Any advice on this would be very welcome.
I don't see any ahem 'blockages' the water is clear. It takes an age to drain down though.
I have tried pouring a bucket warm water from as high as I can manage - numerous times.
Found some sink/drainhole unblocker and shoved that in overnight.
But, it is still not right..
It's hard to get a hand in. As the pipe is quite narrow and bendy - and I suspect that any blockage must be further into the pipes.
I may try the Karcher pressure washer on it later , but that could be quite messy.