Troy wrote:
Just a quickie :-)

In my bathroom, the 40mm waste pipe from the bath (enclosed on the
floor against a wall) passes the wash basin. The wash basin has it's
own 32mm waste pipe and both pipes run (inside the bathroom - no
option) together to the point of exit to the soil pipe outside. Would
there be anything to stop me simply joining the basin waste to the
bath waste pipe by means of a Tee and thus having only one waste pipe
for the remaining run to the exit?

TIA.

According to the charts on the hepworth web site there are max number of
"discharge units" allowed for any particular pipe size at a given drainage
slope.
a 40mm pipe at 1.25 degree slope can have 2 discharge units
a 40mm pipe at 2.5 degree slope can have 8 discharge units.

1.25 degree (minimum) = 22mm/M.
2.5 deg (normal) = 45mm/M

A domestic bath = 7 dicharge units
A basin = 1 unit
So if the slope is 2.5 degree combining the two is just ok.
If the slope is less then no.