When I flush my downstairs loo a small amout of the flush water backs up into the shower tray and then it disappears. What can I do to sort this out? I have tried chemical sink unblocker in the shower plug but that hasn't worked.
Can anyone help? Many thanks.

As a repair plumber I have never recommended using harsh chemicals. In your drainage system. Besides they only seem to work in TV commercials and when you try to remove the water you could burn your hands or damage your eyes. I prefer to snake a drain line m uself but if chemicals are your thing then Try this for your drain. Tonight before bedtime pour 1/2 gallon of bleach down the drain and let it set over night. Next morning, ( and this is important ) flush the mess out with two large pans of boiling water.
The bleach will start to dissolve the hair and make it slippery and the boiling water will melt the grease and flush it away. Good luck, Tom