I believe you live alone. Combi boilers cause more grief when there are multiple users in a household.
I don't think Edinburgh water is hard.
Hard London water, flat (converted house) with 3 people in, combi boiler, no problems.
It helps that:-
* We have good water pressure (the local reservoir is up the hill from us)
* There's only one bathroom (with sink) and a separate toilet (no sink)
* It's a 28kW combi boiler in the kitchen
* The bathroom and separate toilet are next to the kitchen so the longest run of hot water pipework is under 5m
* The only other hot water use (other than the bath) is the tap in the kitchen, everything else (dishwasher, washing machine, fridge) is cold water only
* It's a good quality shower Triton thermostatic bar mixer shower
No shower problems even if the dishwasher and washing machine are both on, and someone has just flushed the toilet and is now washing their hands in the sink in the bathroom.