Thanks for responses.
It is not lamb or other meat fats.
It is a congealed, greasy, thin, near white cement type consistency,
but greasy and sticky on the hands and arms. At its worst, the rods
with claw end break free solid, then semi-solid crap.
I keep the dishwasher topped up with salt which should soften the
water. Nasty, very hard Cambridge water.
There is no feed from a washing machine into this drain.
It has a reasonable fall, but does turn a corner, maybe 45 deg as soon
6 inch after it hits the drain pipe.
I'm not the only one. A colleague across town had similar, and so did
his neighbour. The neighbour apparantly paid DynoRod ~£250 to fix !
But mine seems worse than theirs.

I think I'll try the hot caustic soda every 6 months.

Thanks.