After serious thinking Jim GM4DHJ ... wrote :
I found it hard enough getting traps on one pipe systems without getting
traps on two pipe systems......

Er, some confusion somewhere...

There are two large diameter (9"?) pipes which run through all of the
back gardens of the street. One carries away foul, the other rain
water. We have a shared rain gully at the front, a shared rain gully at
the back. Both have traps.

Then we have three foul water gullies, each with a trap.