Posted: 26/01/2014 at 17:24
If you can find any of the things mentioned in the RHS apple canker control page which Dave mentioned:
they may help a little but there is no cure as such. A lot of the chemicals mentioned are no longer available though. Any time over the winter while the tree is dormant is fine for pruning out the damaged areas which I think is probably your best bet if you want to try and save the tree. Hard pruning will encourage lots of 'water shoots', most of which you'll want to remove in the summer, but you can let a few develop into new branches and fruiting spurs. Advice from the RHS on this (called 'renovative pruning') is here:
Good luck and hope you can nurse it back into health.