I can't do it myself - aside from the plumbing the electrical work scares
me.

I have a mixer tap shower right next to the conventional boiler immersion
heater/tank through a plasterboard wall. So it shouldn't ne tricky.

I reckon the Wickes shower pump is going to cost me between £250 and £300
for a metal one. So with labour and electrical work I'm surely looking at
least at £500 for a shower pump installation.

Because I have an old boiler - 1995 - would I be better saving up and having
a combi installed (special offers claim an all inclusive cost of £1400 for a
cheap one)?

It's going to give me shower power plus energy saving hot water plus,
probably, 5 to 10 years of trouble free fire.