Varying lease demise

I have a situation where 3 leases require amendment to reflect the fact that their parking spaces have changed since the leases were created - the spaces have basically had to be rotated due to planning constraints.

Without going out with a tape measure and finding historical records of how big the parking spaces used to be, it is virtually impossible to know whether we are increasing or decreasing the extent of the lease demise. My instinct is to proceed by way of a Deed of Surrender of Part which will surrender the current parking spaces and then grant supplemental Leases for the parking spaces in their new location which will be a lease by reference to an existing lease.

However, it seems that if one is "reducing" the size of the demise then my approach is correct but if one is "increasing" the demise then the Land Registry will consider that as a surrender of the whole Lease and a regrant.

What I am not clear on is, if there is no change in the area of the demise (or where, as here, it is unclear whether there has been a slight increase or decrease) whether a Deed of Surrender of part followed by a separate Re-grant will be acceptable for Land Registry purposes.

Any guidance would be much appreciated indeed. 

Laura - you are varying the extent/demise of the original leases as the spaces included are each changing. So each will act as a surrender and regrant as the demise is varied from say Flat plus PS1 to Flat + PS3