I had a new bathroom fitted 3 months ago and everything was fine for the first 2 months and then came a bad smell from the shower waste pipe.Have put bottles of the pipe clearing solution down the waste pipe and left overnight but the smell returns as soon as someone uses the shower!Also if I put bleach down the toilet and flush it the the smell of the bleach wafts up the shower waste pipe.The shower tray and waste trap is from JT and the shower waste pipe runs into a stack as does the toilet and sink waste.The plumber who installed it says that the trap under the tray should prevent any smells but obviously it doesn't.Not sure how the trap is supposed to work but there is no water covering the end of the waste pipe so I don't see how there is supposed to be a barrier to prevent smells.How do I solve this problem?