Access to a FH cupboard in a LH flat

Access

A complication arises as the under front step bricked up area is only accessible via the leaseholders ground floor flat
It's unrealistic to expect daily access for example but an entry door from the side path could be built in the brickwork.
Can the leaseholder object on the grounds that I am entering her flat area or compromising security? Could I offer to provide internal locks on her access door to the cupboard as erroneously her meters are located there, an oversight from her solicitors at purchase? She is now adamant that the cupboard area is her meter room and her property that I shall challenge and ascertain with her mortgagee.
Q if the FH at the LR confirms the LH has no case ?
Q. Finally before chiselling out the bricks ahead of the Section 20 notice do I write to say a temporary hole is being made for the builders to assess remedial work for the damp
( next issue is replacing the steps because of ingress) and that her access door to the " meter room" will be secured with an appropriate lock plus key supplied. To allay concerns about the path entry door I shall say it's secured and how.
Q do I have to alert the house insurance company of the hole in the wall and the contraction of the new steps?
Thank you forum