The o ring above, I could find them as a part number & part online, & the price wasn't bad. But the post & package was silly. ( No way I wanted to cover that.)
So I found the size of the o-ring, & found an independent o-ring seller.
I got a bag of 50 (I think) for about 10€, oil / fuel resistant, from place in Ireland.
If it's ok with mbclub.co.uk site rule & terms il find the place & report back.