Started the Mk3 and it idled at about 500 instead of it's more normal 1000-ish. Revved it a bit and I could hear the dreaded gurgling air lock noise through the heater matrix. It gurgled almost continously when changing through the gears. Ran a bit lumpy and stalled once.
Had a quick look under the bonnet, no sludge in oil, no drop in coolant.
I'm sure it can't be anything else, the car has done 141k and as far as I can tell it's never had any engine work, ever.
Still, good excuse to give the head a good rebuild.