Overheating but why?

[VladTepes]

I'm out and about and noticed my temps on my 300tdi defender up touching the red.
Plenty of coolant in the overflow reservoir.
Any ideas?
I'd go to MR Auto but they are closed on weejendw6

[VladTepes]

Oh and fluid in reservoir is hot so there's presumably circulation happening.

[Saitch]

Dodgy radiator cap, perhaps? Try loosening it a little, when cooled.

[RANDLOVER]

I'd check that it is not a worn water pump or fan that's not moving water or air.

[VladTepes]

Seems to be fluctuating again. Will drove home carefully and look into it. I'm suspecting fan at the moment.

[Tins]

Oh and fluid in reservoir is hot so there's presumably circulation happening.
Oh. I was going to suggest thermostat.
So, along with the other suggestions, partially blocked radiator, collapsed hose(s), or of course the dreaded head gasket.
Also, is it actually hot? I mean are there overheating symptoms other than the gauge? iOS, could it be electrical?

[roverrescue]

300tdi defender up touching the red.

Hate to say it
But from above info it’s head gasket till proven otherwise
The normalised gauge doesn’t move till your up over 100
The red is pretty much 125 from memory

Of course feel free to change all the other bits

Or just muck in a knock the head off doesn’t take long