Lease plan does not include the bathroom

I'm purchasing a leasehold flat in a converted house, and the bathroom is not included in the lease plan (created after the house was converted in the 80s). In fact, the room does not exist at all in the floorplan of the building. There are no neighbouring flats that could claim this space; it is only accessible from the flat I'm buying. I'm not sure why a deed of variation has never been drafted as the flat has been sold several times in the last 30 years. The seller now proposes an indemnity policy, but I'm wondering whether this is the right thing to do? EDIT: To clarify, the bathroom is outside the boundary of the lease plan.