Cmac:
What you're describing sounds to me like you simply need to have the sewer line that the drain pipe from this basement toilet empties into cleared with a snake. It's apparant that the sewer line from your house is partially clogged with solids (mostly coming from the kitchen sink, I expect).
I don't know if the codes are the same in England, but over here, if you add a toilet to a basement, you DO NOT have to add a vent stack to the house. You can simply cut a ditch in the basement floor and connect the new basement toilet to the existing drain piping going out of your house.
Now, what happens if the sewer line that the toilet's drain pipe discharges into is partially clogged further on downstream?
Then, when your bathtub drains or your washing machine spins, they discharge water into that drain piping faster than the water can drain away past that partial clog. The result is that the water backs up in the drain piping behind that partial clog, and if it backs up past where the drain pipe to the basement toilet connects, then any continued backing up will compresses the air in the drain pipe leading to that basement toilet.
If the water backs up high enough in the drain piping, then the air pressure in the drain pipe can be high enough to come bubbling out the toilet bowl.
And, this scenario would also explain your observation that it's an intermittent problem. The only time you'll get an air bubble coming up through the toilet bowl is when the water is backed up in the drain piping, and so there's no room for the additional water that would normally go into that drain piping when you flush that basement toilet. Consequently, the water from the tank pours into the bowl and the water level in the bowl slowly subsides as the water in the drain piping slowly drains away past that partially clogged section of your sewer pipe.
Similarily, by morning, the water backed up in your house's drain piping has all drained away through that partially clogged section of the sewer line, and there's plenty of room in the drain piping. So now, when you flush your basement toilet it flushes normally. That's mostly because the empty drain piping upstream of the partial clog can hold more than 5 gallons of water.
You'd have little to loose by just phoning up any plumbing contractor and asking him if a basement toilet will blow large bubbles of sewer gas if water backs up in the house's drain piping due to the sewer line coming out of the house being partially clogged.
And, if he confirms what I'm saying, the fix is to get a plumber to snake out that drain from the bottom of your house's vent stack all the way to where your house's sewer line meets up with the common line from the other houses.
