Pax Guns in Highgate London may still have some Defiant Pellets left.
These are modern pellets made to fit the old guns and work well.

If you lurk around gunshops and Clubs you will find chaps who can service/fix these guns.
I am lucky to have a couple of friends met like this who sort mine.

Sounds as if yours isn't actually broken and from my experience with these old guns we really should not expect great things in power or accuracy.

Nice to own and good ones are an investment.