Unfortunately the seller is the person who mugged me for it, so meeting isn't really an option. Police were apologetic but seems there's little they're able to do.
Very much not official guidance here but how about arranging a meet, or get a friend to, turn up but stay a decent distance and out of sight, phone the busies and describe your thief, say he's waving a knife about, wait for them to show, swoop in and retrieve your bike
If you have a friend willing, get them to turn up and ride off on it. Obviously, it's not always that easy and negotiating to be able to give it a test ride can be a task in itself. If you can assemble a group that are nearby then that would be preferable.
Unfortunately the seller is the person who mugged me for it, so meeting isn't really an option. Police were apologetic but seems there's little they're able to do.