Assuming the other Lessees also own a share and are willing to "comply" (and why wouldn't they, the Lease is getting pretty short), then you shouldn't have a problem. The Freeholders would all have to join in and each separate flat owner would be required to enter into a Deed of Variation, thereby varying the term of years back to the original 99. If one declines to do so, then he/she is really only prejudicing his chances of finding a suitable Buyer and, if one Solicitor is instructed, that should cut the costs down as the Deeds of Variation will be identical (save for each address of course)! Hope that helps