Vokera boiler overheats then switches off?

I've a problem with my vokera boiler whereby it fires up ok when central heating is requested but then the heat doesn't come through to the radiators. The temperature reading on the boiler then goes up until it's about 5 degrees over what the boiler is set to then the green light goes off and it sort of just purrs away but doesn't provide heat. We've had a man out to it who reckons that the heat exchange has gone due to excessive sludge/slime build up and what's required is a power flush costing £400+. Does this seem a reasonable assessment? A build up of sludge and slime seems unlikely to me seeing as the boiler's only 3 years old.