Looking at your picture, MerseyMan, I'd class that as heavily corroded. You will need something more than 0000 wire wool. My suggestion would be a rust remover, maybe from a car shop, then a thorough de-greasing before trying the Bisley blue. One problem is that the gun was never blued in the first place, rather, it was chemically blacked and so you will end up with a mis-match. I think you can get cold blacking products.
@fish5133, yes you can still get lead pellets. (better not shoot fish with them I guess).