Have you taken up the inspection cover in your garden?
Did lift it but didn't get a chance to rod it. The pipe does run in the
general direction of the sink/shower waste but the cover is upstream of it.
I'll do some explority poking tomorrow.

I did stick a drain rod down the WC waste, though I can only get 1 rod
length before if gets stuck, however not sure if this is a blockage or a
bend in the pipe. Still can't get my head round how the sink and WC are
connected though if the wastes run in opposite directions ??

Jim