Jon wrote:

I'm new to all this so please bare with me...

I'm having a problem with a combi-boiler and need help trying to
identify the problem -

The boiler appears to be working fine - i.e. it's able to heat the
radiators and water. However when heating water for the shower it seems
incredibility sensitive to changes in water pressure, if someone were
to turn the tap on in the kitchen the hot water flow in the shower
completely stops. Also, if the heating is on it seems to frequently
cause the hot water in the shower to fluctuate.

I wonder if the hot water flow from the boiler is all it could be. How old
is the boiler? Is your water hard and, if so, do you have an effective
(i.e. phosphor-dosing, not electronic or magnetic) scale inhibitor?

(I'm not sure if it being scaled up would cause the problem of the heating
affecting the suply of hot water, but if it is scaled up you should
definitely sort that out before trying to address other problems.)

I'm wondering is it possible to control the boiler so that it gives
priority to heating water? Also, is it possible to prevent the affect
of turning on a tap in another room?

You could partially turn off the service valve (if you have one - or fit one
if you don't) on the kitchen and other hot taps so that they don't reduce
the flow to the shower so much.