Unpleasant smell

Could it be a leak somewhere? You might need to get a plumbers advice.

I think you might need the services of a professional drain cleaner.

Have you raised the manhole cover . It should be outside somewhere. If its full of sewage you have found your problem.
May be a blockage and you need drain rods or professional help.
This happened to us and also happened to my grandparents when I was a child. I still remember the awful sight when grandad lifted the cover.

When you say downpipe, do you mean the soil pipe stack?

On some soil stacks, instead of being made to be higher than any windows, they are shorter and have a trap valve fitted to the top, when loo is flushed the valve opens to allow the water to drop and wash away, and closes when flush is finished.
If the valve gets stuck open, or becomes slow acting, you would get sewer gas coming out.

Are you on mains drainage?
Otherwise as stree says it may be because you need to have a higher soil stack, above roof level.
Or at the other end, a below ground blockage.
Does your downstairs toilet have the same problem?

Forgive my ignorance, but what is a soil ;pipe stack?

The soil pipe is the main sewage pipe that toilet waste and grey water goes into in the house to join the sewer that takes it away from the house. At some point the sewer will usually have a pipe coming up from it on the outside of the house that goes above the eaves and has a circular vented cap of some kind. Generally referred to as the vent stack, because this is what it does, vent any sewage gas from the sewers into the atmosphere.

Whatever the cause I would call out a plumber.

Thank you MOnica

Get a plumber

I had blocked drains and a smell last week, and it turned out to be tree roots. Had a drains man not a plumber. He had a camera that he puts down the drains.

Oh dear Conni7 that sounds expensive!

My tree roots took £6000 to remove and replace pipes! Thank goodness we were insured.

We had the same and it was a partial blockage in the sewer pipe. We had it flushed out and the problem was solved. Check your household insurance we were covered on ours.

We had the same problem earlier this year. We called out a drainage company who jet flushed the whole soil stack from top to bottom (it serves the upstairs bathroom and two downstairs) and extended its height by about two feet so it is well above the roof guttering and windows. They told us that most older soil stacks are too narrow for todays usage where many homes have more than one bathroom. The work was not as expensive as I expected (just over £300) and we got a two year guarantee.

Grand65 The only way you can get a smell from your loo when its flushed is if it over syphon`s & draws to much water from the wc pedestal & leaves a gap below the water line.Then you will get a smell from the actual wc pan.It`s the same if we get very windy weather it will cause a back draft down the soil stack & suck the water out of the bottom of the pan,leading to smells coming back up.It will even suck the water from your baths,basins ,& shower traps etc.Depending on the age of your property,you could have cast iron or even asbestos soil pipes,these had caulk & rope joints & were not unknown to leak,so its possible to have a small leak from one of those gone unnoticed.Especially if you have carpet in the room.One other thing to check is if you have a flush pipe from your cistern to the back of the wc pan hold the palm of your hand there when you flush to see if its leaking,the cones making the joint there do deteriorate after a few years.Same goes for the modern CC cisterns & pedestals the sealing Doughnuts can leak over time.I hope these DIY tips help you locate the smell.if not keep a can of OUST handy. Kennyh retired plumber.