So after a long drive, starting the car up again showed the low coolant message on my dash. Couldn't see any pools of water, probably because the hot engine evaporated it all, so I topped up and ran the car. Initially it looked like it was coming from above the coolant temperature sensor, possibly a housing but on closer inspection a hose had bulged a lot and was the likely suspect.
The part number is 038121473 and it's a coolant breather hose from the rear of the engine around the right to a metal coolant pipe, and it's about £15.
changing it isn't too hard at all, 30 mins going slowly and carefully. I think the pipe perished over time because it goes right next to the engine and under the tandem pump, hot stuff! It did well to last 140k miles.