You say the replacement is a 'standard' Mira head.
Which, out of the 20-odd heads Mira do, was it that you fitted?
Like I said, not all heads are suitable for use with electric showers.
There original head, started leaking from the seams!
The replacement head was a Mira eco, but with the eco bit which slows water flow removed.
Just ordered one of these from eBay: 180732506088
110 sold, with no issue!..I'll report back once received and fitted.