Background:

I'm based in the UK. I have designed/made a website for a client who is now refusing to pay. I currently own the domain name (+ a few other ones related to the business), and have refused to hand them over until he pays. He is now threatening to sue me to get me to hand them over (presumably still without paying for my work).

TL;DR: Client won't pay for site, I own the domains, he wants to take me to court to get them.

More info:

He recently paid, but then cancelled the cheque before I could bank it (which I understand is illegal anyway). I never signed an IP agreement with him so technically I own all my work. Also, we came up with the brand name together, so me buying the .com is hardly trademark theft.

Has anyone here had a similar experience, or does anyone have any advice they could share?

Any help is much appreciated