Kevin wrote:

At the moment the only thing I think that might work is if I can get a
smaller bore soil pipe which will pass over the top of the lintel. My
builders merchant does a 3" soil pipe but I'm not sure whether this is
permitted for a W/C, whether you can get all the required fittings if
it were (e.g. pan connectors) or even if this is small enough.

We did something similar a couple of years ago.

Building regs said (but dont rely on my memory) then that 3" was okay
for a single WC of a particular type (Siphonic??). I'm not sure if you
could run the shower waste through this as well!

We did put a 4" pipe through a joist but reinforced it with steel
plates. You may be able to do something similar.

Mike