I was quite surprised by the number of upvotes the question got, and as I've now gone ahead and had the panels installed, I'll add a few more details of what I did, in case it is of use to anyone going forward.
First stop was the MCS web site. (this is an absolute necessity, since as @EnergyNumbers says, to qualify for the feed-in tariff the installer has to be MCS-registered)
From this site it was simply a question of calling some local firms and having them come around. I had 6 companies visit, of which 5 gave quotes. (I find it strange that one guy didn't even bother quoting, but I do tend to find this happens with tradesmen.)
With the exception of one guy, who advised to only install as many panels as were required, everyone else said to install the largest array possible. The upper limit to qualify for the feed-in tariff is 4kW, which equates to 16 panels. (Panels seemed to be a standard 250W, regardless of manufacturer.) So I have ended up with 16 panels on the roof.
The quotes were all quite good in terms of projected returns etc., but you do realise that these are "finger in the air" numbers. Nominally, my system will pay for itself in 7 years, which sounded great to me. But how accurate that number is...there are a lot of variables and a lot of assumptions.
The main difference in the quotes was the brand of panels, plus whether to use inverters or micro-inverters.
As for the difference between the panels, it wasn't something I could particularly rationalise - the closest I got was the difference between driving a Mercedes and a Toyota. A big difference in price, but both will get you from A to B. In particular I looked at guarantees of performance levels (i.e. there will only be an x% drop in efficiency over y years) and basically discovered that the panels were much of a muchness.
I was advised by several of the fitters to avoid the generic Chinese panels, simply because there is no comeback if there is a problem. And in fact, when people were talking about the different brands, a lot of them were household names. Panasonic, Hyundai, Benq, for example. The more expensive quotes were with German panels (supposedly the best quality, hence my Mercedes comparison).
Inverters was something I learned about along the way. Each fitter had a preference and I noticed early on that they would be happy to give me all the "pros" of their preferred choice, without mentioning the "cons". So I really felt the benefit of speaking to half a dozen people here.
As far as I could summarise:
- with inverters, there is one of them in the system, it is generally located indoors (making maintenance easier), but the panels basically act in series - so one shaded panel will limit the output of the other fifteen sunlit panels. Also, inverters are not guaranteed as long as micro-inverters, so presumably have a shorter lifetime.
- micro-inverters are a newer technology, exist one-per-panel, and sit next to the panels on the roof, so there are 16 of them and if any require maintenance, its up on the roof. However, because of this each panel acts on a standalone basis. So if one panel is shaded, it won't affect the output of the other fifteen sunlit panels. The telemetry options look good on these too, apparently you can monitor each micro-inverter via the internet. My experience was that an array of 16 micro-inverters added around £1k to the cost of a single inverter.
So obviously there's a bit to think about in this area. For me, my roof has absolutely no shading - the panels will all get the same light - so this drove my decision to go for the cheaper inverter solution.
Last but not least, the price. The lowest quote (and the one I ended up going for) was around £4800. I went through the contract with a fine tooth-comb on this one, as it was quite a bit less than the other quotes. But having gone for it, the installation went very smoothly and the system is working well (Benq panels, Samil inverter, which actually has an ethernet port and provides good telemetry both via the lan and the web), so I am a happy bunny.
The next lowest quote was around £6k (give or take), with a couple more at £7k. The most expensive came in at over £8k. As I say, they were all offering different panels and inverter solutions, but nothing really so different as to justify a £3k price difference. So the moral here is that it definitely pays to shop around.