Hot water problem in shower and taps

ThisIsWeird

A wee test you can try. ID the CH 'flow' pipe coming out from your boiler - it'll be 22mm, and become hot when you turn on your CH.
Ok, you ideally want this pipe not hot to begin with. Either you or hubs goes in the shower, and CH is turned on at the same time as the shower being used - this combination seems to work for you, allowing the DHW to keep being delivered? 
Feel the CH flow pipe as the shower is run. Does it also become hot? 
It shouldn't, not if the diverter valve is working properly. When you turn on a hot tap in a combi, it should fully switch to delivering DHW. The CH should stop. 
If your CH flow pipe also becomes hot, then that suggests - I think - a sticking diverter valve. Running the CH warks for you here, but it 'shouldn't'!

ThisIsWeird

Any update, Turnitaround?