Leather washers are surprisingly good at the job, don't be surprised if it's still OK even after a long career. Back in my lathe days I made an 'O' ring head for a mates HW35 & there wasn't a great deal of difference from the leather original, then he put an Ox spring in it, the less said about that the better.
One thing, £15 is a bit eye watering for a replacement washer, it actually gets a whole new piston head, in practise they are easy to make for a few pence. Item no. 191219758842 on the 'Bay for £3.50 as example gets more than enough raw material.