John Stumbles wrote:
Chasing out the wall below the window cill in my kitchen (c. 1970s
extension) I dislodged the top layer which turned out to be about 3/4" of
plaster sitting on slates sitting on the bricks of the inner course, so
that the slates and plaster covered over the cavity.

I'm not up to date with current spec (chances are it's been changed by
2 jags), but it used to be that cavities had to be sealed with a smoke
(or possibly fireproof) barrier.

the reason is to stop smoke penetration through the cavity from another
part of the building.

The method I've seen most often is to lay cut building blocks on their
sides on the inner leaf to bridge the cavity. A dpc (dipping at least
50mm below the block under-surface) is wedged between the block and the
outer leaf. The cill sits on top of the block.

I've also seen the slate method, which I had to use once due to space
problems over a window lintel (it was approved by the LA Building
Inspector).

Slate is fireproof, but perhaps 2 overlapping leaves would be an
improvement to lessen the chance of smoke penetration. So far as
butting to the outside leaf, I can't recall if I used a dpc, but as
slate is a dpc in itself, so long as no mortar bridge the gap between
outer and inner leaf there should be no dpc problem.


snip

I'm thinking along the lines of stuffing a few inches of rockwool into the
cavity,

If you mean rockwool batts sold for building into cavities, go ahead.
But f you mean rockwool used between joists, I'm unsure about that as
rockwool batts used for cavities have a hardened surface texture which
could be a design feature to limit moisture penetration across the
cavity. BTW do not use polystyrene as that requires an air gap to
prevent condensation in the cavity & stop sweating on the insie wall..

IME you will find it difficult to fill much of an existing cavity from
above, but every calorie saved will help your household bill.

fixing a tanalised batten to the outside wall below the windowcill

Would advise against that as although tanalised timber is supposedly
rotproof it is not water resistant. You'll create more problems than
you solve.

Stick to the masonry or slate solution, avoiding bridging the cavity
with mortar unless there is a dpc stop in place.

and putting some artificial slate

Be careful - what are the water resistant & (fire) temperature
properties of artificial slate?

Real slate can withstand the high temps of a house fire and is nigh on
100% water resistant.

HTH

NB 2 jags' present regs may be different - check with building control
if you have any doubts, better still, someone up to date will post!