The thermostat housing had a bit of steam coming from it and had overflowed. At first I thought the bodge of chemical metal had failed and it had just dumped all the water out again.
But popping off the oil filler cap proved otherwise. I'd checked the water and oil a couple of days ago and there was no mayonnaise or warning that anything was amiss.
We filled the expansion tank and started the car to see what would happen. Lots of water and steam out the exhaust was about the result of that and the gauge rising steadily. That proved conclusively that it was a head gasket failure.
Thankfully, back in February I signed up for AA membership and today was the first time I've ever had to make use of it. The first AA thing to arrive was a van and I laughed when the driver suggested we might be able to put K-seal in, and then he laughed when he saw the contents of the oil filler. He didn't have the relevant A-frame* with him so we waited for a big lorry to take us home instead.
Tonight I spent my last bit of cash on 4ltrs of oil. I'll get the head off, check it for level and use the stuff I bought to do the HLS head on the HL instead now. The only thing I need to acquire is some engine flush to clean out the gunk and I can borrow the relevant stem seal tool so that can be done at the same time.
It's a frustrating set back, but it will allow me to get the engine in tip top health, something I was putting off until the Spring as I'll likely have to do this job in the street as I don't have enough driveway space to use.
On the plus side, the old O series is pretty easy to swap, so fingers crossed you'll have no bother and it'll all go swimmingly 
