Many thanks for your helpful reply - heres the answers inline..

Are you doing this in conjunction with your neighbours?

Yes

Masonry is obviously easier to do from one side, is more robust, and
will resist sound transmission better. *Lay your blocks flat rather
than trying to lay two leaves, make sure that they are well mortared
to avoid air paths, and fill the space between the wall and the
felt/covering in Rockwool.

The party wall is likely 4in single skin brick (possibly 9in although
unlikely), so I'd be building ontop of this if that changes opinion. I
would therefore imagine blocking on edge as per usual, but with single
skin? Would you fix to the chimneys with wall ties to increase
strength?

Is there a more effective way to seal to the slopes of the roof than
rockwool - would you bother cutting angled blocks to fit the spaces,
or is this overkill?

If you want a party wall in studwork that will form an effective sound
barrier, it needs to be done as two separate stud walls, each
independent of the other, lined with 2 layers of 12mm plasterboard on
each side. *A single stud could be used, but it wouldn't form any
serious impediment to noise. *It would also not help you very much if
there was a fire in your neighbours unless you could plasterboard
their side of the stud wall (if they have a fire, the timber on their
side is unprotected and would burn through, collapsing the stud wall).
As you say, you can noggin between the rafters, but fill the space
over the top with Rockwool.

As someone else pointed out, I dont really need soundproofing come to
think of it - just the physical barrier for security and the
fireproofing, so I guess fireproof plasterboard on both sides of a
single skin stud would be effective. Would you bother installing
anything into the stud cavity??

What would your personal choice be in this situation - stud or block?
Which spec blocks would you use btw.

Are there any alternatives to heavy chipboard panels for the floor -
stronger but lighter would be the ideal (whilst still reasonably
priced of course!)

Many thanks for all your help..