Thanks for the replies Knut & David.
I have been mysteriously loosing coolant for a long time. There would be the odd drip on the floor issue, but I have spent several sessions under the car checking every clamp and taking care of the leaks. At no time have I had an actual puddle; well once when I didn't get a new clamp tightened correctly but I took care of that.
The tech doing new tires just changed the oil and saw no evidence of coolant in the oil. I have never seen coolant in the oil either.
I have also never seen evidence of oil in the coolant.
We have changed the radiator out three times. First time was essentially before I bought the car, using an aftermarket aluminium unit. Second time was with a better aluminium unit. This time we re-built a stock unit. I don't see any leakage at the radiator or connections, but will check again.
I had one severe overheating event immediately after the third radiator job with extensive coolant loss. One of the fan fuses had blown due to a servicing error (fan plug backwards). While the car was idling in my driveway it got to about 120 on the gauge and the cap relieved. I shut the car down immediately and refilled / bled it.
Cooling system appears to work correctly, temp range on the gauge normal, thermostat working OK based on gauge movement, fans working correctly. The heater performance is pretty lame, but I understand this is pretty typical; I was planning to separately flush and pressure test the core this winter as I can't see the core clamps to check them.
As mentioned before in different threads, I have been getting intermittent high oil temp warnings only when tracking the car. I have hopefully addressed this issue by changing to different oil grade with this oil change. Unfortunately I was out of town for our last local track day last weekend; idea was to confirm I had the high oil temp issue licked.
After the overheating event I checked the coolant for combustion gas and it was OK. I might want to re-do the test as I was just using a hand vacuum pump to bubble through the tracer and want to make sure I did the test correctly. Is there a vacuum line handy on the V8 to use for the test? My kit instructions say to connect to a vacuum line.
I added UV tracer to the coolant as well to assist in seeing any leaks. I need to get a better UV flashlight, but it really looked OK to me prior to the LOG trip.
On my big LOG trip I checked coolant level twice a day. The recovery system worked correctly, which I have found is not the case if there are small leaks in the rad, hose clamps, etc. However I would estimate coolant loss at about 1/2 litre a day with continuous running.
I asked the oil change tech (not my usual service shop that has done most of the work on the car, but a good guy at our local tire shop) to look carefully for any coolant leaks, and he came up with the head gasket issue. Unfortunately I haven't had time to confirm this, and from his description it might be difficult to see without a lift.
Thinking the best thing to do is to get a better UV light and get the car on the lift at my regular shop to examine the head area and do a compression / leak down. Maybe re-do the coolant combustion gas test first myself. Hoping to get the CEL sorted first as well. Pretty sure I would have to have the head work (if required) done by a shop. I just don't realistically have the tools and skills required. I will be thinking about a more realistic budget.
The used engines seem to be sold, or at least not listed on eBay anymore.
On a lighter note, the new tires seem great.
Any suggestions appreciated.