combi boiler - pressure increases then boiler shuts down

I have a worcester 240 combi RSF boiler. Its always functioned well until a week ago. The problem it has developed started when running the washing machine but is getting worse. Basically running the hot water causes the pressure to very rapidly increase past the critical level (past 2 bar I believe), the boiler then gives an almighty clatter and shuts itself down. I then need to press the reset button to restart the boiler. A couple of other things I have noticed

1) the power light is on but the green demand light doesnt light. But perhaps this could be just a red (or rather green) herring - like most things, I tend not to notice when they are working well and it may be just that the bulb has gone. 2) when the hot water is running the flame does seem very full - but as it is normally not visible behind the outer cover I have nothing to compare with to know what its like when its running normally. 3) I just tried a test to see if the same thing happened with the heating. On this occasion it ran ok, the radiators heated up and then it switched itself off (properly) when the set temperature was reached. But the problem may be intermittent - I then tried the bathroom tap and that was ok too, so thinking I had isolated the problem I ran the kitchen tap - but on this occasion that was also ok. However this morning running the hot tap made the boiler misbehave within less than a minute

Does anyone recognise this behaviour, and have any idea what is happening? And is there anything that I can do, or is it a "call someone out" jobbie?