glowworm betacom 30c HW wont stay on?

When hot tap is turned on water heats up with a good flow , but after 30 to 40 secs gas goes out and after a short time the procedure repeats. I have now noticed that although the heating seems to be ok the boiler seems to be going on and off every minute or so and I suspect a thermocouple could be at fault which works with both events. Any ideas please.