What others have said, also:
Try and find a "stable" company that will still be around in a few years time. Tricky, obviously, but if anything does go wrong it is much easier if they haven't folded.
When we had our panels put in we had a choice of two companies; one was a very small one, and while I trusted their expertise I wasn't certain they had much longevity as a company (it was quite an unstable business to be in when all the tariffs were changing). We went with a much larger company, and they are still trading; we were able to get assistance from them in the summer when our inverter had a brief hissy fit after a power cut. The one-man-band is no more.
Also, ask if they sub-contract the installation. My mum's were put in by a sub-contractor and they really did a rubbish job and clearly didn't know what they were doing. It was eventually put right, but it was a pain. I would say the companies who have their own installation teams probably do a better job.
Check that you like (or don't hate) the look of the panels. Sounds trivial, but they are going to be sitting on your roof for 25 years. Some of them are more obtrusive than others.
Last, see if you can get them to throw in a Bluetooth monitor, which shows what you've been generating day by day, total tariff etc. Addictive viewing (we generated 16kwh yesterday; not bad for February 
).