glow worm 24cxi boiler?

the f5 is that the boiler is sensing it is getting too hot - usually that the water is not taking enough heat away from the boiler. It could be that the flat-plate heat exchanger is partially blocked. You could try a quality cleaner, or take out the heat exchanger and give it a good flush - a bit awkward but worth the 20 minutes if you are competent.
If f5 occurs only when the central heating is running, it may be silted pipework - the pump not running efficiently (try turning the pump up to no 3 if it has a speed control.) Is the pump running at all? - you should hear it running. Try turning down the heat control on the boiler - does it still trip with the F5 fault code. A good gas engineer should sort it quickly, though there are others who have the qualifications but haven't a clue about fault finding and will just keep replacing parts until they strike gold. (your gold unfortunately!)

Mike Whalley
June 2015