GPT seals in Stafford can get the o-rings. 01785 259423. Only trouble is you have to buy a few at a time.
Also if you remove the little metal collar from the 3 part shaft inside the nipple the o-ring problem stops. I did this about 6 weeks ago and havent shredded one since. The tank fills fine doesn't leak and holds pressure. Previously it was shredding every 2 fills.