Glow Worm Combi Boiler Hot Water problems?

I have had a plumber out 5 times recently - to service and fix this boiler. It last had a washer on a leaky part + new air filter £180.

Hot water worked fine for next 2 weeks - now it comes on initially for a few mins then goes cold.

If I 'thump' the boiler in the top right corner it comes on again.

Any suggestions? Worried I am throwing money away on it? Do I need a new boiler?
Thanks Liz