Ok, I admit defeat.

The kitchen plumbing that I've posted about before is getting worse and
worse and despite loads of caustic soda, Mr muscle foaming stuff (expensive
caustic soda I imagine) and some rubbish environmentally friendly stuff
(well, it smelt nice I suppose) we are now at the state where when the
washing machine pumps out water bubbles up through the plug hole :-(

I've tried poking and rodding it from both ends. No joy.

Looking at other houses in our terrace, what we used to have was the
sink drain coming out through the wall, turning 90 deg and then down to
an open gully and into the drains.

I've no idea where it goes then but it somehow goes right under the house
and into the main drains at the front of the house. Unfortunately, the
previous owner built a conservatory on the back covering this gully and
poking springy drain cleaners down the drain I appear to get stuck about
where this gully is/was. As whatever this is bunging it up is now under
nearly 2 ft of concrete/floor I'm pondering giving up on this drain that
has always been a pain and just running a new one. Given some of the
other discoveries I've made in this house I wouldn't be surprised to find
that he has just stuck a board over the gully and concreted it all in, drain
grid and all :-(

I reckon if I could run a new bit of 40mm drain along behind the kitchen
units somehow I could then go through the wall into the understairs loo
and connect it into the soil pipe with one of those solvent weld collars.
Hence my questions...

1) Are there any regs/rules/problems with connecting into the soil pipe
like this? The connection would be within a ft or so of the toilet (a
toilet that we don't actually use but thats not really relevant)?

2) any bright ideas for getting a 40mm pipe fitted behind the units in a
kitchen with out pulling out the units? Lots of short lengths of solvent
weld all glued up sounds plausable and I'm struggling to think of anything
better. Under the sink is a 600mm unit, then a 1200mm gap where the
dishwasher and washing machine sit (so thats easy). Then a small 300mm
cupboard, a fitted oven and a 600mm drawer unit. So it wouldn't be a
hideous amount of joints but still, I'm open to better ideas.

Cheers,

Darren