We have a draught coming up through the shower drain hole. It brings with it a foul smell of the sewer

Sounds like the trap's failed allowing sewer gas to enter your home. Seal off the drain ASAP. I hope the shower's a second floor installation. Then the repair would be a simple one. However if this ism on a slab the shower base will have to be removed and the cement opened up to service the trap. Good luck, Tom