will try to sort some pictures.
When i look down the plug hole of the shower there is water sitting so would appear fine. Cannot get to the trap itself without removing tiles.
All connections on the pan look fine and am sure that the smell is coming from/through the water in the pan, but only when the shower is running.
Is it normal for the waste pipe from the toilet to run into the ground without being vented. There is a seperate stack that is vented but where these meet underground is also where they join the sewer pipe. Could this be the problem?
I suspect that the outpouring of hot water into the unvented stack has something to do with this. You can test this by running the shower for the usual length of time but cold.
Gas will collect in a vertical, unvented stack - but how could it get past the toilet water trap? Possible answer; there are foul smelling gases that dissolve in water: hydrogen sulphide, ammonium sulphide, methylamine (think of rotting fish) - and that's not an exhaustive list.
Are you sure you have a complete water seal in the toilet bowl? When you flush into an unvented stack, it might just suck out a little too much water and leave a small gap for gas to escape. The heat from the shower waste would then expand the gas in the stack and push it up through the gap.
Try this test. Next time you have a shower, pour some water slowly into the toilet bowl to make sure there's a complete seal. Does it smell now?
And a second test:
Don't just get down and smell the water in the bowl. Scoop some out and take it somewhere else to smell it. This will tell you if it's coming through the water.
Been to a job today with the same problem, turned out it was the joint where the pan connector joined to the soil pipe (s trap toilet/ swan neck pan connector into pot soil) this joint wasn't sealed properly. Worth a punt.