Thanks for the pic. So from what you have described the w/m and the sink must be connected together before they run into the manhole. There must not be a problem with the drain going from the manhole (possible part blockage further down) because with the manhole lid off ie. vented there to free air, the gurgling still occurs. Now when the drain from manhole to house was cctv'd and found clear I am surmising the camera went past the w/m connection. Basically all things have been eliminated now except for the "leg" of drain from the w/m and dishwasher. Sorry it's been a bit of a marathon Q+A, but now you can direct your, or the contractor's attention to that leg of drain from where the wastes go into the floor. You may find there is a trap in the drain (look down the pipe in the floor and see if there is standing water, just the same as a sink trap) or it might just be a bend. If you can get the machines out and access the drain in the floor you could then hold a plastic bag (inflated or opened up) as close as possible to the rim of the drain so that when the sink is run you may be able to discern air movement in the drain (in or out) As you have said, the problem has been for about 1 year out of 8 I feel there may have been a long term build up of sludge from the w/m and dishwasher in that leg of drain to where it joins the sink drain. I would suggest jetting the w/m drain from inside the house ( and observing the manhole in the garden ) as the start point for the contractor, whose price seems reasonable.