Well, hopefully this should be the final word on this matter, pending their arrival a successful fitting. Went to Thames Fiat - very knowledgeable and helpful - got told that there are about half a dozen different seals available in different sizes, in both metric and imperial(!). I should say that I had looked on the web beforehand for info on Dowty washers, and you won't be surprised to hear that they come in many different configs of inner dia, outer dia and thickness, plus different 'rubber' compounds. I had actually found a suitable one for the larger pipe but not for the smaller one. So anyhoo, man at Fiat, accidentally or not, let me see their self-made info sheet on these seals, and it clearly showed that the seals for the GP and the 500 are the same, with the same parts numbers - 10877887 and 10877980. They keep them in stock, and shockingly they are a tenner each plus VAT!!! I said that I can get them for less than that on ebay, so I did, and they will hopefully arrive tomorrow.