Shower problems on a Combi boiler.

I have recently had a Mira Fino EV shower (cost £200ish) installed on a combi boiler system. The boiler we have is a Glow worm Swift flow 80 honeywell - about 13 years old, I think.
When the central heating is on the shower works fine (although the pressure is not as good as we were hoping) but with the heating switched off the shower does not get hot. We have resorted to turning the bath hot tap on , then switching the shower on and then turning the hot tap down to a trickle. this usually maintains a hot flow.
I have taken the shower head off and turned the shower on, this results in a maintained hot flow from the hose.
We bought the shower as it was suitable for a combi boiler.
The pressure gauge on the boiler is around 3 bar.
When we had the boiler serviced we were told we lived in a quite low pressure area.
Have we wasted our money on a more expensive shower when a shower fitted to the bath taps would (maybe?) stay hot and have the same pressure.
Any suggestions would be gratefully received.

Sounds like the shower does not give enough flow to trigger the boiler. Or the boiler is a bit slow and sluggish due to age.
Your cheapest solution (now you have the shower!) is to get an experienced boiler engineer to check that it's working correctly. It can probably be improved but especially on that model it's not really DIY.

This sounds similar to what was happening with mine, i have a worcester 28cdi, turned out to be the diverter valve just changed the diaphram today problem fixed yahoo we can all shower again

You seem to have answered your own question.
By taking off the shower head the hot water is constant. Combi boilers are not the best for showers really unless you can get the flow running through the boiler without restriction. The head of the shower in your case seems to be the problem. Buy another shower head that has larger holes to increase the flow. Some of the shower heads have an adjustable lever that can vary the water pattern. With trial and error you will then find a compromise.