My filler cap is clean. Has been since I got the beast.
Check your blow off/vent pipe is not kinked. Especially where it bends to go down through the bodywork and out to open air. Mine was flat and when it did once overheat the expansion tank could not vent. I am thinking if the tank fills when hot It may not be able to flow back if air can not replace the coolant. The tube may be collapsing. I know mine went very soft when hot.

By the way mine was blocked and blew the cap off the tank

Blackburn said:

Might not have made my point very well this is a pic of my rad cap that did not allow the water to be sucked back from the expansion bottle.
Has a nice rubber seal at the inner rad lip but when the water expands and goes into the expansion bottle the thin silver metal seal that seals to top of rad outer lets air back in so water does not return from expansion bottle.View attachment 102370
Parts book show a seal 564999 as seperate part to radiator cap.

Click to expand...

just had a look at you pic of the radiator cap, and the cap you have is for a non reservoir type set up as in the cap is only a one way type, the one you are looking for has a seal around the edge where it contacts the lip.
so change the cap and keep the reservoir up to the half way mark, and it should keep the radiator topped up and all you really need to do is check the reservoir, i usually check both to make sure the cap still does its job.

mine leaks at the timing cover to block and at the bypass hose, and i tighten it up and still leaks at the bypass hose, so i dont bother and i just keep on topping it up every few months as it does not loose too much so i hope this helps.

Tony.

sometimes solvents can cause condensation and white mayo as you explain, i would not worry as you say the oil on the dipstick is fine, you just seem to have that problem, and also some engine will experience condensation inside the crankcase and so on.
just keep an eye on it and see how you go.

Tony.

Many thanks all for taking the time to reply, as a first time landy owner all help is appreciated.
I must admit, I've never had such a love/hate relationship with a car ever!

That's landys for you. Always something going on .