Glowworm Spacesaver KFB 60 cuts off after hot water?

Bought house 1 yr ago, had a problem with the boiler since day 1. The problem is that the boiler cuts out after hot water cycle ends. I hear a gurgling sound from the pipes and it cuts out. If I have the water cycle on with the heating, making sure water stops before the heating cycle I have no problems.

Got CORGI engineer in. First he thought it was the thermostat- turned out ok, pump run off-turned out ok, pcb-turned out ok, pump-turned out ok. All these were replaced but he ended up putting the old ones back in. He also replaced the valve close to the pump (diverter valve??). He rang up Glowworm who say there could be a blockage in the system. I have put in Sentinel X400 in the tank on his advice. Been told to leave it for 4 weeks.
Been a week, still the same problem.

It has a 3 port or a Y plan valve system?? The engineer can't recall seeing a bypass system on it.

I would be grateful for any words of wisdom on this matter. Thanks in advance.

(Called engineer in for service of the boiler- told him of this problem- had several visits from him to sort out this problem-service not done yet!!)