The prices seem about right, so it would be better to replace it.
It is hard to see how it was working with only one of these broken, nevermind all of them?
The property is under offer, and will be sold in a few weeks, so replacing it is a big pill to swallow :-(
£1000 for a new Ravenheat or Ferroli or £650 to make this one good as new?
Do you have the make and model - perhaps you can source the parts cheaper although they do sound correct
It is an Alpha 240E
I have found alphaboilerparts.co.uk
Would any of these problems show up in a Gas Cert done at end Nov?
An alpha 240E, ok not the best but quite reliable
I suppose you are just unlucky
An easy swap with something cheap just to get you out the door?
The problems are stopping the boiler from working, while the boiler was working these items would not have been faulty - so there is nop clue until it goes wrong
The diaphragm would show as the hot water not starting every time or having to run the boiler faster to get the hot water to fire the boiler