I had a quick look in the other day (for the first time) and saw that the bottom of the horizontal pipe has about 20mm thick deposit, not soft, shit like stuff but a hard crusty layer which I can chip away, a bit like a weak mortar bed. Clearly any lack of smoothness in this run will slow down the flow and allow it to build up. I seem to recall reading about self cleaning velocities in pipes carrying mixed liquids and solids and it sounds logical.
I got a 1m steel rod and hammer to it but that was as far as I could reach.
We do have fairly hard water so is this likely to be some form of horizontal stalactite / stalagmite being formed over the 30 years since installation?
Having chipped it away, I can't provide a photo of course. DOH!
I wonder about fitting a temporary plug where it joins the stack and pouring brick acid down the loo to dissolve the rest over night>
PS It is not a viable proposition to re-plumb to get rid of the horizontal run and it still works after 30 years