Intermmitant warm water from Halstead Ace boiler?

Re: Halstead Ace:
I paid a plumber £141 to diagnose a faulty diaphagm.
I waited over two weeks for him to return with the part (And a further bill!) to no avail so I bought a diaphragm kit for £35.83 from PTS which was a rip off.
It was a doddle to fit even with no instructions.
It was indeed split but replacing it made absolutely no difference!
I still have hottish water for a 30 seconds and then the boiler turns off and cold water eventually comes out for 30 seconds during which the boiler fires up to produce the next 30 seconds of hottish water.
Can anyone advise?
So far I've spent £400 in labour charges and the boiler has been used only to test with erratic results from various plumbers who are qualified but clearly just guessing.