These vary in price from 30p plus vat upwards, plus shipping.
Robsport seem the most sensible but you don't get shipping costs until after the order is placed - although I didn't try phoning during office hours.
£2.81 for shipping a £1.08 oil filler cap gasket, seems fair enough.
Unfortunately I found this out too late so......
Shortly I'll have several fuel pump to block gaskets for sale for £1.50 each including shipping!

The minimum order where I sourced them was £6 including shipping, so I expect to see about ten of them. The worst I came across was £7.20 shipping on a 35p gasket.