Hi Charlie, and welcome to the forums!
I feel I should mention my own "source" of backing tracks as well. I get all my tracks from this site: www.licklibrary.com
It's not a free site though (cheapest option is a subscription at around 12 euro per month which includes unlimited backing tracks and tons of song/technique lessons) but the quality of the backing tracks are completely unmatched by any other source I've found. I realize this might not be for everyone, but personally I'm lucky enough that this kind of expense is easily justifiable for my big passion and hobby.
PS. As a disclaimer, I'm not involved in any way with running the site... this is 100% a recommendation from a happy customer
BTW, I just remembered: I'm pretty sure they have a 14 day free period which will give you access to all backing tracks and online lessons (that's how I got hooked
). They have a quite good "Walk of Life" backing track that you could get for free that way if you want... streaming it right now actually.
PPS. After a blurb like this I guess they really should