Thanks for your question.
Why are you being asked to grant a deed of variation? It would appear on the information provided that there is some sort of issue with the demise and the ground floor.
Sometimes problems can go unnoticed and are only discovered years later.
If there is a clear defect and you refuse consent then the matter could be referred to the lands tribunal for rectification of the lease. If the matter went to tribunal there would be cost implications and the tribunal may award against you on costs as a hearing could have been avoided if you consented.
I am certain that as part of agreeing to the variation the other party will cover your legal costs, if not they should be. Your fees should be paid.
As to the issue on reward, this will be difficult to argue if there is a clear defect, why should you benefit from a defect.
Also consider that rectifying the defect will benefit your freehold interest. It could potentially affect marketability of your freehold interest if the lease is defective.
I hope this response is helpful. Please remember to accept /rate in order for me to receive credit for my time in responding.