Does anyone know where you can get single impeller pumps? I've not found
any on-line and wouldn't mind getting one for our HW system in general.

I believe someone recently recommended the Grundfos UPA 15-90 "Home
Booster".

The other manufacturer normally recommended is the Stuart Turner. They do
quite a number of single impellor pumps, from their entry level Showermate
1.2 (around 130 quid) to a Monsoon 3.0 (around 290). Obviously, there are
twin impellor versions of these, so check carefully.

Christian.