Thanks everyone!

I think I'll give it a go with my cranked chisel bit, maybe running a drill
up first to clear the plaster.

One thing I maybe didn't mention is that most of these chases *may* contain
one section of 20mm round conduit for ELV, so the brick needs to be cut too
(plaster is about 10mm thick). The cranked chisel will probably do that
better under the coving.

However, if it's all too yuk to do that, I could replace the 20mm with
2x16mm oval. It's to do with the fact that the ELV circuits will be wired
later and will involve upto 4 cables of more than one type (eg Cat5e/aerial
coax/SELV control flex/loudspeaker cable) so I need to allow a bit more
conduit space to make sure I can get stuff down easily on demand.

Mind you, thinking about it - as I'm grouping 2-4 euromod positions with a
double 13A socket which itself may need a vertical drop, it might be easier
just to knock all the plaster off for the full width of a double backbox,
lay 4x16mm oval onto the brick and replaster one wide chase rather than
several little ones... I think most of my plaster is deep enough for oval,
but as usual I expect to be caught out...

Thinking aloud...

I'll come back when I tried some next week...

Cheers

Tim