Hi Chris, I seem to have missed your thread but Phil has been looking after you.
Here are my thoughts on coolant/mixing issues on your QP;
First and foremost the coolant tank is the first area that should be checked. They crack and are cheap and simple to replace. Look for coolant staining underneath and white stress fractures that open up and close depending upon the tanks temperature.
Rear hose/back of engine. Don’t tend to weep but total and instant failure (ask Stevo) resulting in disablement of the entire coolant system.
Heat exchanger. Situated under the plenum in the V of the block, this is a radiator that flows both oil and coolant in the same rad. Internal failure is common and results in oil and coolant mixing with no visible signs of leakage. Look for oil in your coolant (greasy white film) and vice versa.
Head gasket failure is extremely uncommon. A simple sniff test will prove any exhaust gasses in your coolant.
Radiators, uncommon unless damaged by a stone or the like.
Minor leaking. There is a complex network of pipes around the engine with no end of jubilee type clips. A good visual of them all is required and a pressure test is recommended. Get the car up to temperature so the thermostat is open and full flow is achieved. Remove the expansion cap (exercise extreme caution) attach your manual or air driven tester and charge the system to 15-20psi then wait 15 mins and look for coolant drop in the expansion tank and where it’s exiting.
Mayonaising. Coolant in the oil giving a thick white residue. Collects around the oil cap and underside of the cam covers. If the car has been sat for a while and a sniff test passed and the heat exchanger is good this is most certainly due to condensation and nothing to worry about. We see this a lot.
Hope this helps and you get your issue resolved painlessly.