This is the link (
http://www.clsdirect.org.uk/legalhelp/leaflet1 1.jsp?section=5&lang=en) but have copied the relevant bit below...
If someone damages or interferes with your property, you can sue them for 'trespass to goods' or 'conversion'. You can also sue them if they refuse to give your property back to you when they are not legally allowed to keep it.
When can the police take my belongings?
The law gives the police wide powers to take and keep people's belongings. In particular, they can take and keep property if they have a good reason for thinking the property may be the proceeds of crime (such as stolen goods) or evidence of an offence.
The police are allowed to keep your property only for as long as they need to. This can include when they want to use the item as evidence at a trial - though they shouldn't hold onto something if a photograph or a copy would do. They can also keep something when they want to work out who the true owner is.
How can I get my belongings back?
If property is taken from you after you have been arrested, it should be given back to you when you are allowed to leave the police station. The police can hold onto it only if they have a reason for thinking that it is the proceeds of crime or evidence of an offence.
If you sue the police for interference with your property and you win your case, you can expect to get compensation, as well as getting your belongings back. If you just want your belongings back, you can take out a summons in the magistrates' court under the Police (Property) Act 1897.