The internal walls in my 1960s house are all built of red coloured, hollow
blocks which are very hard. I always have to faff about when fixing to
them - sometimes you drill into a solid bit, other times a hollow bit, so
you end up messing about with different fixings. Sinking back boxes for
sockets are a nightmare - it's not deep enough if you don't cut into the
block, but if you do, the hollow bits are too deep, then there's hoizontal
solid bits to cut out.

I don't know the name for these blocks, but does anyone have any advice on
how to work with them?

Peter.