FYI the ceiling joists are not
resting on the wall plates, but are constructed so that they are fixed
to the (substantial) rafters which sit on wall plates. This means
there are sloping edges to the ceiling that I havent yet worked out
how to insulate behind the lathe and plaster, due to lack of access -
any ideas how to finish these?

I'm puzzled by the construction here... how come there's no wall between
you and the neighbours up there, but apparently there *is* a finished
lath and plaster ceiling over the rafters?! Or am I missing something?

David

Sorry if I misled you - it is an unusual construction I think. For
clarification - the roof rafters are not covered at all. About 4 ft
from the end of each rafter ( poking into the outside, the ceiling
joists are attached. This gives nice high ceilings with sloped egdes
towards the walls. It is only this (approx 2-3ft) of the rafter that
is lath and plaster (on the slope) then it flattens to the ceiling
proper, formed by the lath and plaster on the joist. The remainder of
the rafter goes into the loft, uncovered, as a continuation of the
rof. Hope this helps!