Thanks for that Slick132. I've consulted a few lawyers who are experts in lease extensions and they had various opinions about this. But finally I found one who said that there was a way of extending the lease without needing the mortgage provider's approval. As I understand it you create a head lease (i.e. an additional lease). Then when you come to re-mortgage or sell, the two leases are merged which is apparently a very simple process. So that's is what I'm going to do - hope this helps anyone esle in the same position.

Hi mrw I have a Woolwich mortgage and I’m wondering if I will have a similar problem to you. My flat is a leasehold and I was about to get a lease extension which requires Woolwich to approve via a Deed of Substituted Security. Apparently getting this deed is very routine – but I’m wondering if Woolwich will take this opportunity to make me change to a repayment mortgage. If you making a simple phone call can make them switch, then surely a change of lease (albeit a longer lease which makes their loan even more secure) is definitely a change of circumstances? Given your experience how likely do you think it is they will claim this is a change of circumstance? Also has anyone extending a lease has had this problem – i.e. the mortgage company take it as an excuse to change the goal posts? Thanks all.