Gap in firewall

Do_i_Diy

After just having a survey done for the sale of our home he has discovered a gap of about 6" in height between the top of the firewall and the eves in the attic to our neighbour. This is right in the top of the eves and about 3 meters in length. It will need to be fixed before the bank will release the mortgage. Any suggestions as to how to fix this, it is a really awkward place to get to, so I was hoping blocking the gap with a rockwool type material or filling in with plasterboard.
Thanks

System

Do you know what rating you need?
If you go to the British gypsum website they have quite a good search faculty for finding what the build up can be and the white book has all the guidance on how the details are built to maintain the integrity...