Looking for a bit of advice please.

My mum has an electric shower (Mira Supreme, which according to the web is
either an 8 or 9 Kw, I have yet to go and look at it) that has been in the
house since new build in 1994. She is telling me the 'pressure' has
decreased over the last few months, it still works OK to have a shower but
"not as powerful as it was", She has removed and cleaned the shower head
which makes no difference. Is it likely to be furred up and if so is there
a cure, apart from replacement. If she goes down the replacement path can
you get 'powerful electric showers or are the dependant on your incoming
water pressure? I presume you cannot put a pump in the circuit because of
pumping mains water. Assuming the cable is the correct rating is it worth
putting a higher Kw rated shower in it's place, and will this create the
feeling of more pressure? I hope that makes sense to everybody, it does to
me but I know what I am trying to say!

Cheers

John