Hi Sophie,
Firstly, your service man sounds like a right p***k.
You say your boiler fires for 20mins, does your heating system get hot?
You also say, your boiler then turns off, does it need to be reset to relight?? as in gone to lockout.
This sounds like a possibe control problem rather than a boiler issue, go check your room-thermostat when the boiler stops and then turn it up to full...see if it refires....
Let me know how you get on
Tony, February 2008