On Sat, 23 Mar 2019 13:07:58 -0700 (PDT)
Keefiedee wrote:

We have a problem with a drain smell in our cloakroom, which is
fitted with a shower. I realise that it is possible to fit a
non-return valve to the outflow, but access would be very difficult.

I'm guessing that shower doesn't get used much and the trap has dried
out. Just run it occasionally to make sure the trap is full.