I have read the various topics on smells from sinks and have as far as possible eliminated the various causes that have been suggested.
On further investigation the smell appears to be coming from the (concrete) floor around the pipe connection from the sink. The pipe (32mm) feeds through a black rubber pipe collar (?) with a hole bigger than the 32mm pipe The gap between the pipe and the collar was 'sealed' with some sort of rubbery gunge but this seal had a small hole in it. I removed this gunge (on the basis that I assumed it would be easy enough to replace!). I was then able to disconnect the downpipe and looking down through the hole in the collar could see a larger white pipe which connects to a corner brown pipe that takes the water away in the direction of the cistern.
I have sort of convinced myself that I have found the source of the smell and by carefully resealing the gap between the downpipe from the sink and the collar my problem will be resolved. Is this likely to be the answer to my problem and if so is there a better way to go about it apart from applying more gunge?