How can I fix Potterton Combo100?

Hi All, the above boiler in my flat has started to play up. It is about 5 years old and has been fine up to now. The problem is it locks out after being on for a short while due to I think overheating. Also once on, it won't turn off even when you turn down the thermostat. I have had a guy look at it and he says I need a new pcb and a new heat exchanger or a new boiler.

Anyone out there know if this is the case or can offer any assistance?