Well, an enormous amount of advice, and thank you very much to everyone who has contributed. The one thing that i am absolutely convinced of after reading it all is that I shall not be disappointed that I have at last achieved the status of "Sport owner". It seems that I can't go far wrong if I try an original spring and seal, and if I don't like the result at least I shall have a base to work from in trying other measures, so I will give that a whirl to start with. As I said, I should like to keep it within the spirit of its time if possible - I wonder what tinkery new owners in the seventies did to improve it? And, not that i can work to their standard, but what was done to an Airmasters tuned one?