"The Natural Philosopher" wrote

I tend to agree. The bits alone will be at least a grand if there is
nothing there right now..thats for a combi type system: More if its mains
pressure.

Probably £1500 of labour by the time its all squared away.

Agreed.
Having recently had some upgrade work done to a larger system, I was also
thinking 2.5-3K.
However, this will depend on:
a) The availability of plumbers in your area and therefore their rates (I
contacted about 20 around Hull and only had 5 visit, of whom 3 bothered to
quote).
b) The quality of components you (or the plumber) choose to fit.

Phil