Jude,
I had a totally open loft between my neighbour and myself in a 1890 house (always strange when opening loft hatch to see next doors xmas decorations). Could not walk the entire length of terrace as their was a chimney stack every two houses (but these did have a small gap so in theory I could have walked and crawled entire length).
To remedy this and to ensure their was >30mins of fire protection, ontop of the party wall (the top of which was exposed in the loft) I used two methods:
- used breeze blocks on their side, mortared together in easy to accces areas, or
- made a stud wall, filled with rockwool, sheet of standard plasterboard (as red fireboard too heavy to get in loft) on each side in harder to access areas (for example your 'rear/ out rigger part' of the house).
Decisions were made based on ease of getting materials up into the loft through loft hatch, and width and consideration of weight of block ontop of party wall (stud wall where party wall was only one brick wide).
Note that no builders were willing to do the work as access was very difficult.
Not sure if a party wall agreement is needed, but as I did the work with the neighbour we did not bother.
Some mortgage companies will not provide a mortgage for the buyer if buyers surveyor discovers that there is no dividing wall. So it is useful adding one now, well before you consider selling.
I also put a smoke alarm in the loft.
SFK