In my experience they are all pretty much hit and miss.
Some fit like a dream, Grundy's V3 replica went on without even needing a trim, where as another could require being chopped up, filled , sanded, re-shaped etc.
Best ones we have come across are the Aerokit ones, sold plenty and had no issues at all so far, have a look on here :
http://www.tarmacsportz.co.uk/products.aspx?CategoryId=43917caa-7861-49d5-a6fd-a668cc92c13e
Drop me a pm if I can help further
Cheers Chris