Yay Britannialloy pump turned up and it hasn't got any dents.
Washer is past it though.

But my local oldtime hardware shop has a box of leather cup washers that fit
50p each.

Damn fine shop: https://www.beltonandslade.com/
84 Wandsworth high street.

Edit: Exact pump size 19": 49.5cm - 46cm including hose head.