Big mistake buying a Worcester

A qualified gas fitter installed my Worcester Greenstar 30si boiler in July 2012. It has never worked properly. Worcester said it was not under warranty because a Honeywell thermostat was used. They sell their boilers without a thermostat so how can they say this?!! I changed the thermostat and the boiler still does not work. It comes on and when it reaches 21oC it goes off. The radiators go cold, I get cold, but the boiler does not come back on again. I have tried turning the thermostat up to 30oC but the boiler still does not come on. I have reset the boiler and the thermostat umpteen times and it makes no difference. Some days, the boiler does not switch off and the temperature goes up to 26oC. I then have to turn the boiler off because it will not switch off itself. The gas fitter tells me that having fitted two different thermostats and still having a problem, it is DEFINITELY the boiler that is faulty. But, it is not covered by the warranty, it is less than 2 years old, and I now have to consider replacing it. My advice is do not buy Worcester, the after sales service is appalling. They are not at all helpful and do not care about their customers.