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
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.