COMBIcompact wall mounted boiler problem?

Hopefully somebody out there can help me, my combi boiler is heating hot water for approx two minutes then the burners cut out. If i play with the DHW microswitch under the boiler i can get it to work again? the central heating works fine, but the burners cut out after 2 mins if we run the hot water.
We live in the centre of Bath, Somerset. It would really help us if someone can put us in touch with a local Vaillant specialist. Thanks in anticipation Pete