Month on and I'm still experiencing regular HW hot shoots, but I know have a solution which limits the impact, so I'm happy!

Basically I've now got an Opentherm Gateway monitoring the boiler, this is integrated into Home Assistant and I've got a Home Assistant automation which switches off the HW if it detects Evohome requesting maximum output from the boiler, the hot water temp raising, but no temperature update for 10 minutes. This works perfectly, so when an overshoot starts to happen, rather than before where the boiler would run at 100% for an hour (because the a HW temp update got missed by the controller), worst case this now only happens for 10 mins, meaning the temp gets limited to between 55-60c. The automation, can tell when the boiler is calling for maximum heat, I've also programmed it to reinstate HW schedule when the temp falls below the Evohome HW lower threshold.

Happy days, but frankly this shouldn't be required!