I've run all the hot water out of the cyl but the boiler behaves the same - it reaches 72C and then burner goes off even though cold water is returning.If the flow and return when you run your Central heating are much more than 20*c difference (which I suspect they will be if you know theres no hot bypass water getting back up there) ie a temp difference of 30+ degrees C would be clear that there is circulation issues.
I understand that slow circulation would cause this and yes that is what Dan first said.
Dan - when you ask "What do you know?" - a helluva lot more than me! I'm a diy guy trying to understand the problem.
I just don't understand where the restriction could be. I've tried a new pump but no difference.
Agile - thanks but I've watched a Glow-worm Ultracom working yesterday which Is also set at 72C and the burner stays lit as it approaches the max temp so continues to put heat into water.