We bought into Solar Thermal in the 1980's and after moving installed again with Smart Energy who went bust. This has caused us some challenges to get a company to repressurise the system. After using Solar companies for Solar PV quotes and the governments mess with FIT tariffs we gave one last try with Spirit Solar. They were exceptional and so we now have Solar PV as well.
If you have any doubts about a company just do not go with them. If they are not honest, straight forward without hidden extras (scaffolding, strengthening roof, monitoring costs, maintainance etc) be very careful. Solar PV and thermal is NOT cheap so go for the best you can afford. Check any projections offered with someone who understands spreadsheets and realistic issues like Inflation, Fuel inflation, realistic returns etc.