2 questions,

The mid position valve has stopped working. I’ve got the actuator off and its doing nothing so that’s buggered, but the valve only has a very narrow range of movement, is that normal or should it swing through quite a large range? Its moving 30deg or so (tab straight ahead to slightly anticlockwise). The brass valve is cheap but I’d rather not have to drain the system for the 4th time in 2 years!

The waste pipes on the back of the house for the kitchen and utilities are all crumbling (UV? Or the OH has a bad habit of pouring white spirit down the sink despite repeated explanations that modern water based paints won’t wash in it anyway!), is all waste pipe created equal or has someone cheaped out, do I need to buy something in particular to avoid it happening again or should all plastic pipe be suitable?

Drayton mid-position only moves 8-10° Honeywell maybe has a wider range??

MuPVC has good heat and UV-resistance. Might be solvent-weld only, rather than push-fit, although open to correction on that. Otherwise try a decent brand of above-ground pipe (polypipe, hunter, etc)