I experimented a bit and it seems it is much easier to re-thread through 25mm oval conduit than 20mm round anyway, and almost half the chase required, only about 12mm to completely recess the oval in the block(to ease any future order from on high to replaster).
The 35mm back boxes are in the block 25mm, and I managed to do this carefully without smashing the back of the block out. I've a feeling that the metal backbox is as strong as the brick I've removed, but I've left a big gap between the AV wall plate and the next back box to ensure that I haven't compromised the inner wall.
I'm just deciding now weather to fill the small gaps behind and around the wall plate with cement for strength or plaster board adhesive. Both seem pretty strong though.
I managed to use three vacuum cleaners to catch the dust from the chaser, one connected to the chaser and the other two held wither side by some reluctant volunteers! This kept the dust to a minimum, although dust sheets were still needed over everything.
Anther tip was using a 2kg SDS with a scutch chisel. My only experience before was with a 5kg SDS and my feeling was that this would destroy the wall, but with the 2kg and scutch I could finish off the chase and backbox detail without much effort, lighter too!
The wall chase chisel for the SDS didn't seem much help and had a tendency to dive in. Without the grooves already cut by the wall chaser, the chase chisel would have also been really difficult to use, as the profile is curved and the inside corners of the chase would still have needed lots of work. Maybe the more expensive square section chisel would have been better, but in the brittle block, the scutch would work easily as well for both the sides and the back.
Thanks again for all your help. I'll post a photo or two when I find my camera in all masonry dust!
