loo flush coming up through kitchen sink

when i flushed the upstairs loo dirty (!) water came up through the kitchen sink. Downstairs loo and sink working ok. This is a house i've just bought, so don't know history of the drains. Had Dynorod out and we couldn't find a manhole cover for him to jet. He went off and we got the grass cut and hired a metal detector to try and locate the manhole. No luck. Neighbours were giving us advice (all different), so we lifted some of the patio. Still nothing. Called the council,and the water board and no-one knew where the cover was. Eventually the water board came out to look, and it turns out we don't have a man hole cover, so no-one can do anything anyway! What will I do now? We have located the stack, but the water board guy couldn't get the cap off it to have a look. Should I hire a snake and put it down the sink drain? We have rods, but husband says can't rod a non-existent chamber. Dynorod said to get someone out who can suck and blow at the same time(!) but can't get a hold of the guy he suggested. Please help!!

you could check that the sink waste hasn't been pushed too far into the soil stack spigot.......maybe?

is that the pipe that goes into the side of the stack? Can I just pull that back a bit? I'm scared there will be a huge backload of nastiness that'll gush out if I do anything. The dynorod man made it all sound very complicated and dangerous, so am nervous! The kitchen is a pit, so I'm not bothered about a bit of mess but he made out there could be hundreds of gallons of pooey water in our house and next door, 'ruining the solent and stinking for years'. What if I pull it out completely by mistake, will all these things happen?

See which of your neighbours has a "manhole cover". If you are in a terrace there may be one to several houses. It will be useful to lift more than one to see if they are flowing freely or if one is full.