Had a look around for possible causes of coolant loss. Felt round all hoses, felt dry, no coolant crust visible. Temp sender was seated nicely, again bone dry. When I opened cap there was a whoosh and the level rose back a fair bit. Noticed a few black specks in the new bottle (bits of inner hose? Old radiator?)
Checked oil filler cap, all good, no signs of milkiness
Fans working ok, both kicked in when I switched on the AC.
Coolant constantly flowing back in from top of reservoir too, so I’m guessing no blockages either.
Is this all pointing at knackered water pump?