The wall on the path up to our house is crumbling away which is an issue as its on a bankside. Currently the path isn't wide enough to get a car up, but if say 'somehow' the path magically became 2ft wider when the retaining wall was rebuilt could there be a planning issue?

There are no kerbs kerb to drop or pavements to cross-over as their are non on our side of the road.

Loads of people up our street have widened their driveway from one car width to two car width, and from what I have read this is allowable with planning approval.

Strictly speaking there probably is a planning issue, however do you get on with all your neighbours? If so I’d just do it and make sure you don't upset anyone, if not you might have 'problems' if they decide to kick up a fuss about it. 😉