You have a drip somewhere on the DHW pipework or taps
If the drip is not obvious then I would strongly suspect mixer taps especially shower mixers
It has been known for the paddle that separates hot & cold in mixer taps to fail causing cold mains to back up into the boiler via the DHW pipework & cause OVERACTIVE PREHEAT
there are 2 solutions
1 replacing defective mixer taps (once identified)
Or
2 fitting a non return valve on DHW outlet immediately coming out from boiler
Q
September 2015