Any plumbers about please?!

[Jooni]

Can anyone tell me what this pipe behind our toilet is? It's open at the top and we periodically have foul sulphurous smells coming from here which heavy bleach down the toilet does nothing to shift.

A bit of googling tells me this might be a soil vent pipe? And if so, shouldn't it lead out of the roof of the house and not just into the cupboard behind the toilet?

Looks like someone previously had a washing machine installed there. measure the diameter of the pipe in inches and buy a cap from b&q.

As above it's a washing machine stand pipe. The curved pipe is a trap to hold water and block the smells from the drain. If it's not in use the trap dries and if the wind comes your way you get the smell from everyone's toilet.
Cap, cling film or even some vegetable oil (non evaporating) would help. But if you won't use it just get it removed.

Ah thank you both! Will get something to cover it. Can't remove as it's a rental property and LL is very strict. Thanks!

tip a jug of water into it. At the bottom is a trap. Like all U-bends, the water forms a seal to prevent odours or sewer gas drifting out. With no washing machine draining through it, yours has dried out.