Gave it a good blast with a garden hose then rodded it too, and have got it pretty clear.
Thanks very much Hugh you were spot on - it's an interceptor - and the ceramic lid had fallen off and down into the main gully (though thankfully it hadn't broken). Will maybe see if I can get it taken out if I get someone to help me make a fresh connection to the drain.
View attachment 290367
View attachment 290368
A nice clean job. You could eat your dinner off of that.