I fitted the shower and the shower pipe I fitted is boxed in, inside the bathroom. On occasion a foul smell comes up from the plughole and stinks the room out, I know it's because I connected the down pipe to the waste pipe (which I have easy access to). Can I fit some device to stop the foul smell coming up the pipe.Cheers.
There is clearly syphonage going on as water falls away it creates a vacuum behind it and sucks the water out of the trap which I assume you fitted.
If you connected the shower outlet to the soil pipe then everytime the loo is flushed this is what is happening. You have two solutions.
1. Disconnect the shower pipe from the main soil stack and run it into an adjacent hopper (assuming there is one that the bath and basin discharge to). Don't connect it directly into the bath or basin pipe as you'll only get more syphonage. If no hopper take it all the way to ground level.
2. Fit a HepVO self sealing waste valve as SB suggests. I'm not a great fan of these (although they are the only solution in certain installations) as there is too much opportunity for them to block up with hair, soap scum etc etc particularly if they are inecccessible after installation.