Vaillant VCW GB 242 EH Central Heating Overheat?

Hi,

I'm having trouble with my boiler (actually my Gran's - shes just been quoted £1650 to have it replaced but i think the guy is trying to fleece her, please read on) seems to be that the central heating fires up, the radiators get too hot to touch and the Flame Supervision Device then trips, i'm thinking that the CH overheat thermostat is knackered and thats going to be my first port of call for a replacment part, does anyone else have any experiance of this fault, any advice gratefully recieved.