Thanks for the reply. I am going to get builders in to quote. Judging from the replies I have already had I am relieved that nobody thinks it's subsidence.
And as far as I'm aware it's not a major job and can be carried out with the doors in situ.
Bear in mind the same will apply to all the windows but probably not an issue, I'm only aware of two people who had work done and that was only to the patio doors.