chaffateaux and maury 80-se, 80 light ?

we've had numerous problems with our boiler over the past few months, most recitified-luckily through insurance!!

latest problem is this - when heating on, we have no problems at all. But when heating is off, and we run hot water for a long time (bath/shower), when we stop the water, the 80 light flashes. We reset and then it flashes 70 & 80 (no flame detection). When we reset again its fine. Any ideas whats wrong?

:0)