I had the same low pressure problems recently. The shower had been in and working fine for 14 years but the low pressure light came on recently and the water reduced to a dribble.
Checked and cleaned the filter but no change.
A friendly plumber advised me that it might be the pilot valve causing the problems.
Went on the net to
Brilliant site! Had all the parts diagrams for all shower types.
I was able to order part online and it arrived 3 days later.
For my shower(MIRA SPORT 8.5 1992- 1996
the part waS:
419.87 PILOT VAL/SWITCH ASSEMBLY
Total cost £39 including delivery
I HAVE SINCE FITTED IT AND THE SHOWER IS WORKING PERFECTLY!