[simon clemens]

Not sure if you have tried this yet, but here is a suggestion that has solved the same problem for me in the past…

Remove the radiator cap and with the engine running have a look into the radiator coolant, if there are bubbles in the coolant you will probably have an internal leak on the head gasket from the water passages into a cylinder. Will need to remove the head in this case.

You could try and re-torque the head, this has worked in the past to stop small leaks.

Do you have access to a cooling system pressure tester. With a pressure gauge?

Simmo