Thanks guys for your very helpful advice, it's very much appreciated.

I'll give it a try with the 'mocked up' installation as suggested by Mike
and see how it goes. The longer run is from the bath, the shower will tee in
relatively close to the ultimate connection to the soil stack.

If there is a problem, I can stick a stop end in the tee connector and get
the SDS drill out!

Cheers,
Dermot