There are two types of deposit protection scheme. If your landlord is using a 'custodial' scheme (rather than an 'insurance' scheme) your deposit should be automatically transferred to the new owner by your current landlord completing a form within 14 days of the completion of sale.
Take a look at the paperwork related to your deposit's protection. If it refers to the Deposit Protection Service Limited, your landlord is using a custodial scheme and you shouldn't need to do anything.
If the paperwork refers to Tenancy Deposit Solutions Limited or to the Dispute Service Limited your landlord is using an insurance scheme (which he shouldn't really be doing if he was aware that he might sell the property prior to your original tenancy ending) and he's obliged to ensure that you get your deposit back as soon as he longer holds any interest in it. (But, of course, there's nothing to prevent you reminding him about returning it, as early as you like).
Chris