Hi Beth - the key is in the original 'demise' so if that didn't include the loft space then you have a 'new lease' scenario
That's achieved through a Deed of Variation in the vast majority of cases as nothing else is changing but as you are varying the extent it acts as a surrender and regrant of the old lease and a new leasehold title is created
I suspect they are now looking to add the loft space rather than there was a 'slip up' in the original demise so the
Deed of V route would be appropriate