So as the pre warmed water leaves the boiler and is replaced with cooler system water what happens to the temp differential if there is no bypass?
It increases resulting in kettling!
Theres loads of apollos round here and they all have a bypass! I live near to what I am told is europes largest housing estate and many apollos are there still providing good service with a moderate appetite for thermocouples, pilot burners and stats. Plus of course the aforementioned DIY tampering problem to which I still get the odd call
Blacon in chester?
Well it used to be [not sure if it still is] Ingleby Barwick in Stockton on Tees made famous recently as a hot spot for Identity theft.