Use either caught.co.uk, dropsystem.co.uk or domainrecover.co.uk (book with them all if you can), these are the only public facing systems that have a chance against the private catchers. Bear in mind that if the domain is a premium generic you will have no chance of getting it with a public service.
If they're booked there you can try dropcatcher.co.uk but they're not what they used to be.
Avoid any other companies as they will have virtually no chance.
Grant