Registering a lease

We have inherited a flat, we thought it only had a 40 yr lease but today found out from hmrc that the lease was renewed in 2001. So the lease was renewed at that time to 999 yrs. Unfortunately this was never registered by the solicitors dealing with it. How do we proceed, payment was made to extend the lease, hmrc recognise this. The solicotor dealing with the estate had no idea. Any advice will be greatly received.

David - our online PG explains when and how to register a lease. I would strongly recommend using a conveyancer to assist and to complete the application to register as appropriate https://www.gov.uk/government/publications/leases-when-to-register-pg25