I'm fitting a mira excel shower but I'm confused. The instructions say
a drop tight pressure reducing valve (PRV) to be fit if the supply
exceeds 5bar maintained. I suspect that my pressure is quite high (it
comes out of the cold tap a lot faster than the hot tap) the hot tap is
fed by a worcester 25si which I understand provides hot at 2.5 bar.

The combi boiler won't affect the static pressure of the water. The fact
that the hot tap comes out slower than the cold tap tells you precisely
nothing about the pressure. For that, you need a pressure gauge. If the cold
is above 5 bar, fit a PRV.

Alternatively I've seen some pressure equaling valves he

http://cgi.ebay.co.uk/Pressure-reduc...QQcmdZViewItem

Should I try one of these instead?

Your shower should have one of these. They are frequently part of the shower
valve. If not, fit one, whether you have a PRV or not. It will significantly
reduce the effect of water use elsewhere in the house on the occupants of
the shower.

Christian.