When buying a leasehold flat you should seek advice from someone who is legally qualified like a solicitor or conveyancer.  They will deal with all the issues of buying the flat. 

It is important that you find out what charges are due as you will be liable for these on completion of the sale if they have not been paid by the vendor.  The solicitor will do this for you.

Under the terms of the lease you should formally advise us that you have purchased the flat.  There is a fee for to register the assignment currently £35 plus VAT totalling £41.12.