Variations in heat from taps and shower?

Roy, You dont say what model Mira shower you have.
It should be a thermostatic type. It sounds like a modern boiler so ensure it has a modulating Burner / Gas valve (Ring Ferolli) It probably has . Modern thermostatic showers require the Hot supply to be 10 degrees higher than the required Blend ie what you shower in . If your hot is only 55 degrees and your showering in 46 - 48 you may not get proper control. Post a question to the corgi lads. I suspect you should have at leats 60-65 drgrees from you boiler.
Check that your supplies are not crossed on your shower valve . It should be hot on the left cold on the right. You should be able to feel. If your installer is coming back or if thats you isolate the apropriate services and remove and clean the service filters on the shower valve. Good luck

Alan
December 2008