There's a bit more to it than the previous two answers. Whether you get interest on the deposit will depend on two things.
Firstly, if your tenancy agreement was signed before 6 April 2007 then you will probably only get interest if it states that you will in the tenancy agreement. Check your agreement. To be honest, most agreements wont cover this and so you wont get interest.
If you signed after 6 April 2007 then it's very probable that your deposit will gain interest because it will, by law, have to be held in an approved deposit protection scheme and most of these will pay interest. The rate you get will be quite low as interest gained on the deposits is used to run the free schemes.
Contrary to R1Man's comment, most deposits are actually returned in full - 70% of all deposits are returned with no deductions. Only 11% of deposits are not returned at all - and this can be for a variety of reasons (damage to the property, unpaid bills or rent, or indeed greed by the LL).
Hope this helps.