"Nick Brooks" wrote in message
om...

I'm planning to take the waste from the shower and sink together down
a 40mm pipe and the bath down another 40mm pipe. At what point can
they join the 110 waste from the loo before it goes underground?

110mm below for a 100mm stack, 250mm for a 150mm stack.
I think if you use two standard connectors you should be ok,
but the soil junctions with holes for 40mm pipe look illegal.

what are the relevant building regs?

on www.odpm.gov.uk IIRC.

Does this look like a good plan?

You could squeeze the 40mm waste between the
soil stack and the wall, to join into the bath waste.