So after getting back from B&Q with the fittings, I started removing the old wash basin and fitting the new.
I wanted to take everything off the old basin - taps, plug hole, etc.; clean them up and add to the new. Unfortunately, the plastic nuts and washers had perished and disintegrated when I removed the taps. Quick trip to Homebase to get some new taps (I think my wife wanted new anyway).
All done and fitted now. The water to the basin is still turned off to allow the sealant to dry. We’ll see how it turns out tomorrow.