In reply to Rob Exile Ward:
Your rights are governed by the contract. Read that.
Assuming it says nothing about returning the deposit to the tenants (which would be highly unusual) then your seller is totally out of order; they have contracted to transfer the freehold to you with all its attached rights, and the deposit is one of those. If they didn't want to be potentially liable on the tenancy agreement for your breaches of it they shouldn't have sold the freehold, and if they are really bothered about it then they should serve notice under section 8 of the L&T (Covenants) Act 1995.
As someone said, the deposit should be in a protection scheme and can simply be transferred to you.
Your solicitor really ought to know this stuff, it's true, but don't be too hard on them - most residential conveyancers wouldn't, and it's not their fault if the other side make unreasonable demands at the last minute. And believe me, the fees conveyancers charge to do the job properly and know everything they 'ought' to know are not high at all in relation to what is involved. This is why it usually isn't done properly.
Nothing obliges the tenant to give you a new deposit if the present landlord returns the existing one to them. Do not give way on this; you are in the right and if you do it may well cost you money.
jcm