This might be a bit late, but I thought I'd stick it up anyway. A perosnal mental exercise if nothing else.
WARNING: This post is very long. If you have anything better to do, you might want to do it instead.
Now, consider I'm a newbie myself. This is just my two cents, and it may or may not be a ripoff. I've been reading up for years, but I'm a growing virgin, so to speak...
Fluorescent lights have the wrong spectrum for growing, as I'm sure you've heard. This is, of course, not completely accurate, fluorescents can centaintly be used, but in terms of efficiency, they aren't optimal for any purpose. During the vegitative stage, plants grow best with lights that emulate the bright-white or blue-white light of sunlight early in the year. This will promote stocky growth and short inter-node stem growth, yielding the bushy plants we indoor growers love to have. During the flowering stage, the deeper orangish-red light, similar to sunlight from a sun lower on the horizon, works best.
The thing is, no light source yet invented has been able to closely match the light spectrum of the sun. It produces a very bright, very wide spectrum, that changes based on time of year, angle in the sky, etc. So for indoor growers, we just try to find the best tradeoff to stimulate the proper bio-chemical signals, yet also be efficient and economical. To do this, we use fluorescents mainly for the earliest growth periods, during cloning or seed germination. We use Metal Halide lighting to promote good vegitative growth. We use High Presure Sodium lights for the flowering stage. Or we use some combination, or a spetrum enhanced bulb to try and get the best of both worlds.
But these artificial light sources still don't work like natural sunlight. sunlight, as I mentioned, produces very broad, even light level across the visible spectrum, due to the imense number of different nuclear reations that occur under it's surface. Aftificial bulbs will usually generate light from a small number of reactions, such as the ionic charging of a selection of metal ions, gasses, fluorescent compounds, and the like. This produces the varied and very different spectrums of the many varieties of bulbs out there. None produce quite such an even field of visible light.
Fluorescents, for instance. The light of a fluorescent spectrum certaintly not a smooth gradient of tones. Fluoerescents produce the greatest amount of their light in the blue and green areas of the spectrum. This gives them that whiter that white lighting aspect that makes them popular for the clean-looking spaces like supermarkets and such. However, compared to sunlight, they produce a lower level of other visible frequencies, such as red-orange range. This in itself reduces their usefulness. The green area of the spectrum is used only somewhat in photosynthesis. As most plants are green, if you've ever noticed
that shows that green light is being reflected. A considerable amount of the fluorescents' output could be in this range, reducing the amount of usable light out ofit's total output. Also, red is an important spectrum to have during all stages of photosynthesis, and gains condsiderably during the later flowering stage. Both HM and HPS produce greater red frequency light that that of fluorescent bulbs, Im quite sure.
Recently, people have been revisiting fluoescent lights due to a new wave of full spectrum lights. One one hand, 'cool' and 'warm' fluorescent bulbs have been offered for years. In most cases, this was simply a coloring that was added to the glass to filter out the light spectrums that made things, especially photographic subjects, look pasty, harsh, or otherwise unpleasent and cold while under fluoresent light. This did little to improve on the actual output of the bulb.
Some bulbs are now claiming a spectrum optimized for horticultural purposes. I'm not an expert on these light sources, but it's possible they could be much improved over older bulbs. Much as HID makers 'tweaked' their bulb design to produce greater dynamic light range in Son Agro, Hortilux, and AgroSun bulbs, some fluorecent light makers are changing the way their fluorescent bulbs work, using a different mixture of gasses to produce spectums more condusive to growing. These are likely best realized for vegitaive growing, perhaps more lettuces(sp?) and herbs. For tomatoes, flowers, and other plants valued for their fruiting bodies (like our own special 'herb'), we still need good output in the red-orange range is very valuble for good growth.
Another issue. Efficiency improves quite significantly based on the wattage. I don't have numbers specific to fluorescent, but I can paraphase ones from papers on MH and HPS lighting systems: The ballasts used on these systems do not actually draw the same amount of power as the bulbs they are designed for. I.E. a 70 watt HPS of HM bulb will lose around 35% power. In other words, it will draw 95 watts, but only 70 will be used by the bulb for light production. A 1000watt bulb, however, is 7% ineffiecient. It would draw 1070 watts of power for that 1000 watt bulb. If you only needed 70 watts of light, then it wouldn't make sense to use a 1000 watter, but it usually makes sense, in many ways, to buy the biggest bulb you can reasonably make use of. Not saying that there are not benefits to having smaller bulb sizes, AND I'm not saying antything about the ballast efficiency of fluoesecnt light sources, but 22.5 fluorescent hardly make sense at all. Of course I'm not suggesting I though you were seriously considering that option, but still. Besides, I sure it would be dangerous to try and cut one of those things it half, and I somehow doubt it would still work.
I respect a desire not to put too much money and effort into something. Trying to balance this aspect has kept me from accomplishing much of anything. If you feel like buying a cheap bulb and doing it half-assed (half-arsed? is that used over there?), have-at. It's better than wimping out and doing nothing at all. I've many times heard how inspiring it is to see your fisrt plant sprowting form the ground, and how it influenced someone to really put some effort and money into it. So just get to it. Don't go itno it unawares, of course. A good read-up on materials, lights, nutrients, legalities, whatever, is certaintly a good idea. But it's too easy to put it off forever if you're waiting for the most perfect light EVER. Just consider you get out of it what you put in to it.
Now to practicality. I'm a yanky, forsooth, and know little about the difficulty in obtaining things across the sea... but for your operation, there are a few possibilities that seem good.
I've recently heard about something called the 'Envirolite.' It's a compact fluorsecent type bulb touted as having a full spectrum, and runs at 95 watts, putting out something around 8900 lumens. For your two plants, one or more of these might be good enough. At ~$100(US) they aren't that cheap, but sound nice, also usable in smaller spaces (1 1/2 foot or so) unlike many fluorescents, at 4 foot long...
Another possibility is a small HPS light. In the US, they can purportedly be gotten at Home Depot, a home improvent/hardwarey type store. They run about 100/150 bucks US, and are 70 watts. You might be able to get 150 watt systmes as well. They are sold as 'home security lights.' I don't know how availible this kind of thing would be over in european countries. Lastly, of course, the internet. Ebay is where I've been looking, you can go with a gamble for less or nearly sure for more. If you have access to a credit card, ar a money order, etc., you might consider this option. You did say 'financially limited', I don't know whether thats six quid or £100. Either way, you have to grow within your means. Buy what you can afford and get started, I suppose.
I realize this posting is disgustingly verbose. I'm truly sorry, I have not ability to be quick and precise when posting this stuff. That said, I hope it has been slightly helpful, or illuminating (no pun intended), to some degree.
As always, the very best of luck to you...
Cheers,
Cassius