Q.Canker on apple tree bark.
Our apple tree has canker on a large branch and near the ground on the main trunk, which looks really bad. I have a feeling that I might be fighting a losing battle, since I have not kept a proper eye on it, although I have followed pruning advice for a few years.
Shall I cut my losses, remove it and get a new one next spring?
I have attached photos.
Many thanks,
Ruth Hobson.

I agree, if treatment and pruning has not been successful at this point, I would look for a replacement.
Talk with your County Extension Office and get a list of Apple Tree varieties that will do well in your area.