going to display my total ignorance of plumbing by asking this question:
if
mains pressure is high, why, according to an adviser from Tapstore.com,
are
wall mounted taps designed for 3 bar pressure not suitable for combi
boilers, as he says that such boilers require 1-1.5 bar?

The "advisor" is getting confused. The combi boiler's secondary circuit will
be pressurised to 1-1.5bar. However, this is an utter irrelevence. The tap's
requirements will be based on your mains supply pressure. 3 bar seems a very
high amount to require, but you may find you have it anyway. You will need
to a pressure gauge connected to your existing kitchen tap to determine what
you have available. If it is above 6 bar, you should install a PRV (pressure
reducing valve), as excessively high pressure can cause problems.

Christian.