Well, my wife's cylinder head is off now (2001 1.4 Rover K Series), but it turns out there is no obvious damage to the gasket - in fact the only thing I can see is a 5mm section of silicone bead is not bonded to the gasket, but the bead section is completely intact.
So, is this the cause of the oil/water mix? Is it poossible that this can happen elsewhere, i.e. around the water and oil pump?
The only bad news I can see is that the two exhaust valves on the cylinder at the other end to the cam pulleys are bright white, whereas the rest are grey or black. Could this be pointing to the cause, or is it more likely to be because the engine has started to overheat in the 3 mins it took for her to bring the car back?
When I inspected it 10 mins after it was brought back (engine off), there was a little steam coming out of the oil filler cap and a little mayonnaise around the inside of the cap (and upon later inspection in the pipe running from the cam cover to the inlet manifold), but not much to see elsewhere. And of course there was no coolant visible in the header tank.
Anyone had an experience like this?