It's taken me around ten years to solve this one. So many folks complain of it and it's usually only the downstairs loo.
Trying to be green, I fitted 'reduced water usage' flush devices to my loos. In retrospect, the problem started shortly afterwards. Tried everything. When I eventually bought a set of drain rods, I noticed masses of compressed toilet roll came loose. So here's what was happening; a smaller quantity of water wasn't flushing the toilet paper from the pipe (about six feet away). This partial blockage started to back-up until it reached the toilet's U bend, where it now allowed the water in the u-bend to slowly flow (osmosis) into the sewer. This causes the water level in the U-bend to drop below the level it needs to be to stop the sewer gasses flowing into your loo. The answer is simple: fill a bucket with water and pour it, as rapidly as you can (without it overflowing) into the bowl, to clear the partial blockage. Maybe do this a few times. Now, use a 'long-flush' every time toilet paper is used. If the problem persists you'll need to clear the blockage using rods. My problem ceased immediatly. This problem doesn't seem to occur with upstairs loos - I guess it's because gravity assists. My son's new house had the same issue - long flushes solved it. Good luck.