Thanks for all of the responses - much appreciated and gives me a lot more confidence to tackle the job. I hadn't appreciate that new taps are still sold in imperial.

In terms of getting the old tap off, I can see that is very likely to be a problem. Tricks I'm aware of: penetrating oil, light tap with a hammer, heat, Please let me know if there's anything else.

One quick question on that: am I likely to be better off with a fixed or adjustable basin wrench?