Thanks for the suggestions - I found there's a few companies that make up rattle cans and bought one, and some lacquer, primer and various degrees of wet and dry. That's on it's way so I spent an hour last night with a wire brush on the drill, a file and some sandpaper
I now have bare metal. Like an idiot, I didn't take photos of the start point but when I took the file to the flap, I ended up using an edge and in so doing lifted various layers of flaky paint and some primer in chunks. However, bare, it's far smoother than it was and assuming I can get the prep right and handle a spray can...
You do of course realise that this job is displacement activity: my heater levers need replacing...