Jonty , if you can get it to Liverpool , there is a Royal Marine armourer who is very good with restoring flinters, The Cap and Lock Gun store in Liverpool
he has a phone listing but you'll have to look there ( BT blocks directory look ups from outside the UK )
i've not dealt with him for 4-5 years but he knew his stuff and was rather good price wise too ( compared )
i currently use http://www.peterdyson.co.uk/
as i have a deal with them ( i made repro arms here and they are a agent) but mates in the UK say they are not cheap to deal with , i dunno, so be careful money wise, they can and will make the spring to suit, but what they'd charge i dont know .. if George from cap and lock do it it may be way cheaper , but he was a small one man show back when..
good luck and cheers