Alright, lets all relax. I thought I was the one supposed to be getting stressed out by my heating system!
Tbf to James, there's been a lot of suggestions, particularly on the other thread, and im sure the correct answer may be in there. If the answer is changing a setting on the boiler or adjusting a valve, im happy to try it. If its changing out a part or adding something, im incapable of doing it. Given ive already wasted money on solutions which haven't worked already, even if im given the right answer, I want to be sure it will work, and ill still need someone to come and do it. From my perspective, if the guy offering the solution is giving me a guarantee it will work or no fee, I dont see that as a problem. As long as the cost is fair for the work to be done (and within my budget!).
Some more info. Boiler was heating downstairs and cylinder from about 12. Hot water was being used to shower the kids at the same time. I turned heating off at 12.30 to see how long the cylinder takes to heat up. Boiler continued to fire for about 20 mins, modulated right down to barely audible. Flow temp 70 degrees, return creeping up past 60. Then boiler shut down. D2 is set to 5, so anti cycle time in d67 is 2 mins. Boiler has now been short cycling for the last 40 mins.
Ive increased d2 back to 20, but as suggested previously, maybe I need to increase to 60. Once the boiler gets in this state, it doesnt seem to recover. Im guessing I need to allow the return temp to drop sufficiently to allow it to fire up and modulate down past the 50 seconds. Tbh, im disappointed it turned off when it had modulated down.another 10 mins at 70 degrees, I believe cylinder stat would have been satisfied. HW target temp is 60 degrees, roughly 7pm on the megaflow stat.