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describe the awkward place. also, any external interceptor/inspection pits with covers, or external soil pipes? as ever, a picture could be useful
rive_gauche-
It's about 2-3m from the outlet, there's probably another 1-2m before a point where I know it's not blocked. Kitchen sink, shower and share the same drain, seemingly, as they're blocked too. The blocked drain feeds into another (probably the house's original) drain, which is also fed by a sink that is working. Shoved a rod up that for 4m with no effect, so these two feeds probably come into that at either side via T-junction. I reckon the blockage is under the floorboards, so my hero of a dad is going to help me pull some up on Friday and see what we can see. I don't think it's a collapsed pipe as there's standing water in the bathroom sink still.
Polygon
I've already tried rods, thinner flexible cleaners, caustic soda, acid. The problem is that the blockage is in a really awkward place, so access from outside is near impossible, or I have a collapsed pipe.