Sabre 24 cuts out when heating water?

Hello,

I have a Sabre 24 combi boiler. It works fine for the central heating, but when I put on the hot water tap, it will fire up for about 30 seconds, the water becomes hot, but almost immediately it will cut out. The boiler will then try to fire up again, doesn't quite manage it and the dreaded red light comes on.

When boiler isn't in use, pressure is a constant 1 - 1.5 bar which I think is fine.

Short of standing at it constantly switching it off and on whenever this happens, which isn't good for either the boiler or me, I don't know what to do. Can't even get enough hot water to wash the dishes without "fixing" it at least 3 times.

Has anyone else had this problem? What could be causing it?

I'm hoping it's not a serious problem as the heating is fine, it's just the hot water.

Please help!