Hello Gareth,
If your master cylinder has been previously re sleeved in stainless steel, then all you will need will be the seal kit. Rover Classics or J R. Wadhams are but two businesses that can supply.
If by chance your master cylinder has not had such treatment, then you will need to find a business to undertake the re sleeving, and I would imagine that most brake places would be able to assist, either doing the work in house or sending it away. Either way you will still need to obtain the seal kit.
Ron.