Glow worm space saver boiler problem?

I have a glow worm space saver boiler and the hot water cylinder is making noises.
The thermostat is only engaged for a short period of time before the hot water is blowing through the vent pipe into the tank in the loft and out of the over flow pipe. I've changed the thermostat and motorised zone valve, but still have the same problems - any ideas?