Sorry to hear about your situation, Graham.

I assume you're talking about a website? Do you host it? If so, I'd imagine that the client hasn't paid their hosting bill? If that's the case, they'd probably have no legitimate claim if you took their website down for non-payment of the hosting (at the very least).

I'm not an attorney so I can't give you legal advice and while I've been involved in similar situations, I've not been involved with them in the UK.

Going forward, however, I can tell you some ways that I've avoided having to deal with a single deadbeat client for years:

• Sign an agreement. Not having one can create the biggest nightmares known to man.
• Get milestone payments. I always ask for a deposit. 10-25% is usually reasonable depending on the type of project and total project value. And be sure to get payments at each major milestone along the way. This will alert you to payment problems early and before you've done all of the work.
• Don't deliver or deploy until you're paid. All of the development agreements I sign dictate that I don't deploy until the client's final payment is in the bank. This is easy when dealing with code, harder when dealing with creative. Nonetheless, if you structure your agreement so that the client doesn't get the final, usable work product until after they've signed off on it and you've been paid, you won't find yourself in a situation where they have everything and you have nothing.
• Don't assign intellectual property rights until you're paid in full. All of my agreements state clearly that all right and title to the intellectual property I produce is not assigned to the client until after the balance due is $0. In this way, if I ever found myself in a situtation where there was a non-payment issue, the client would not own the intellectual property that I created and I could take action based on those grounds.

Hope this is of some use to you. Good luck!

Pu this into their stylesheet

html {display:none;}

That should get their attention.