All of the above advice is first-class, one we got to the bottom of the situation. However you've now put a sting in the tail - you want to sell the lease, which is presumably worth thousands, when you've been discussing with us a couple of hundred Buildings cover, apparently obtained under false misrepresentation by the freeholder.
Personally I'd be taking the longer term view that you now need to rid yourself of the lease, and I'm not sure it's worth kicking up about the Buildings insurance since you will have to answer truthfully the buyer's solicitors standard questions once you've got a buyer.
That's my angle on it, but interested o hear from NJ and others who have put in great contributions thus far.