apparently the TDi was indicating up in the red on the motorway.  Mother and
Father off to the far east (Norfolk) by way of an overnight stop at
Thrapston travlelodge.

just had another call: they stopped and let it cool a bit, then put the
heater on hot, and continued steadily, at which it was fluctuating up and
down from hot to below hot.

when it first showed hot, it still had coolant up to the level, or nearly
so, and the ancillary belt is present and correct.  Shouldn't be an iffy
water pump as it was replaced not long ago.

They're under instructions to see what the gauge says in the morning when
the engine's definitely cold - could be an iffy gauge or more likely iffy
sender.

failing that, sticky thermostat, perhaps, not opening fully?  Either that or
the rad, I reckon.  It's not been overheating locally though, so far as I
know, and it didn't overheat 'til well into the journey today.  Or maybe a
collapsed rad hose, but I think they're all reasonably tidy.  Spose it might
have an air lock in the engine, but I don't see why given that the coolant
wasn't low.

They've not been thrashing it.  

Any of you anywhere near Thrapston?  There's no way I can get to look at it.
They're supposed to be at a funeral in Norwich a 11 ish tomorrow morning,
which don't leave a lot of time though.  My inclination would be to pull the
thermostat on the offchance, and see how it went.  Better it runs too cool
than too hot.

If anyone feels like doing a knight in shining armour bit...?

failing that I suppose they can always get onto the RAC if it recurs
tomorrow.