We were renovating an old bathroom and wanted to "do our bit" by installing a solar water heating system. The house has an existing combi boiler and we were suspicious of solar water systems that use vast hot water tanks that you then have to heat with gas or electricity in the winter, so we opted for quite a small hot water tank and an extra tap in the bath for the solar hot water. We also fitted valves to allow the hot water for the house to come from either the combi or the solar tank as we chose. The system is like this:
We bought most of the components from TheSolarShop who gave pretty good technical advice and seemed to have keen prices. Standard parts like copper tubing and fittings came from B&Q and ScrewFix.