In order to obtain the best performance from your solar thermal system you should have a bi-annual service. However in the first few years an annual maintenance check is not really required
As system service should always include the collector surface is clean, there is no corrosion, sensors and fixings are properly in place etc.
The most important thing to check is the transfer fluid; the system needs to be completely flushed out and the transfer fluid replaced every 5 years.
The most important check is one you can do yourself; leaks! If there are leaks of anti-freeze this will have a strong smell and will corrode even roof tiles.
Additionally you should check that you are getting hot water on the solar pipework on sunny days