Hi,
They are:
Part No. 68-0588 6mm x 10mm or 1/4" x 3/8" or BS010
The best you can buy for this application will be FKM (FPM, viton/fluorel).
I got mine from:
Eastern Seals (UK) Ltd
Unit 3 Sovereign Business Park
Jubilee Industrial Estate
Ashington,
Northumberland, NE63 8UG, UK
Best wishes...