It's all good news so far.
Pulling the glow plugs and injector seats didn't reveal any water in the cylinders, and so I started the engine, it started first time so it's clearly not cooked. I had to put 6L of water into the coolant tank, clearly quite a bit had leaked out during the last run.
There was no sign of steam or smoke in the exhaust, which is good. Also good is that there was no sign of water in the oil, although the engine oil is somewhat burned and needs replacing. Also wouldn't hurt to replace the thermostat I guess. No sign of oil getting into the coolant either which tends to indicate that there's no break in the head gasket.
The mystery is that I can't see where the water is leaking out. Running the engine up to temperature didn't reveal any obvious leaks, and there was no obvious water missing from the coolant tank after I shut it down. The mechanic suspects that there may be a leak in the exhaust gasket which could push exhaust gas into the coolant tank therefore driving all of the coolant out of the release valve at the top.
The mechanic's booked out until end of year and so I'm going to ring around for an emergency mechanic to at least have a check over to see if I've missed anything obvious.
Now where did I put those spare oil filters ...