I'm doing a 24" 52mm dia hole with a just bought diamond
> core bit from Screwyou. Had a go but it seems slow - but it
> is limestone which seems harder than anything, harder than
> granite?
Limestone is much, much softer than granite...
Any tips on using it? Presumably not sds rotary/hammer but
> rotary only? Fast or slow? Pre drill with long bit?
Your problem may be that the soft limestone is clogging the bit, especially if the stone is at all damp. Perhaps withdrawing the bit frequently and cleaning it might help, I don't know.
J.B.