Share of freehold

As flat leaseholders,  we are buying the freehold to the building. Attached to the building is an unoccupied shop which will remain the property of the current freeholder. It is clearly marked on Land Registry entries in red, showing the boundaries.
The Freeholder's solicitors say a new plan needs to be drawn up to update the boundaries. They want to charge us £500 for something we consider unnecessary.
Additionally,  it will also delay our purchase of the freehold - the same solicitors want to charge us an extra £2,800 for this!
What action can we take? 

June - any conveyancer will charge a professional fee to cover the services offered etc
From a purely registration perspective you are purchasing part of the freehold so a form TP1 with suitable plan will be required - see Registering land or property with HM Land Registry: Change the registered owner name - GOV.UK re forms and Preparing plans for HM Land Registry applications (PG40s2) - GOV.UK section 3 for quality of plan needed for a Transfer of Part.
I don't know what 'action' you might be thinking of but if you are concerned re the professional fees being quoted then you can shop around for quotes if necessary