Sewage Smell by Shower Drain - Just had plumber round

[Charnwood]

I don't think the OP has actually stated whether there's water in the trap.

Ensure trap is full of water up to normal working level.
Re-assemble trap.
Flush toilet, listen for gurgling at shower waste.
Check if water still in trap.

If all's OK repeat with all other fixtures with waste outlet; basin, bath, bidet.

 

[gigz]

Another test is, wait until you get the smell again then stick a thin piece of wood down the trap to see if its full (pour water down, do the same again and compare measurement)