That's a tricky one. Could you tell the police it's been taken without your permission? If you've still got the receipt as proof of ownership then they'd possibly call round and have a word ... but knowing how they tend to work, they're more likely to say "that's a civil matter" and hang up on you.
I'll be honest, you probably need a lawyer to start sending letters and try to get a court order for the return of the guitar. That way they can probably be made liable for all your costs. And if you start action against them in Newcastle then the case will be heard in Newcastle and they'd have to travel up from London to contest it so it'd work out a lot cheaper and easier for them to just give you the bloody guitar back!
Usual disclaimers about me not being a lawyer apply, obviously. But I'd be talking to a solicitor now.
