Lots of people are being overcharged by firms employing high-pressure salesmen. If you get a mail shot and you send back the pre-paid reply card, you waive your legal right to a 'cooling-off' period. There have been several cases of firms declaring bankruptcy and then setting up under a new name, with no legal responsibility for outstanding guarantees and service agreements.
Always get a variety of quotes before you decide - and don't just go for the firm with the glossiest brochure. Never sign an agreement on the spot - take your time over the decision.
The bottom line: A good, family-sized solar water-heating system should cost around £3500. A really simple one can be as little as £2000. A really smart, high-tech complicated one could cost £4000-5000. A competent DIY-er could buy and install his/her own system for about £1500.