Your engine has either a current or historical oil cooler leak issue where the gaskets fail and allow oil into the coolant. The ensuing blackening of the header tank is the result and that’s why yours looks like it did. Gone to a good home: 2011 4.4 TDV8 Vogue SE Buckingham Blue with Ivory and clear glass = "Rory"
New Defender D300 90
1974 Series 3 Lightweight = "Millie"
My preferred specialist: www.glenrands.co.uk
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I have a bill for the oil cooler at 9,000 miles ago last year, so I presume the garage did not change the tank at the same time. It's certainly not in the bills that came with the car.

Did they change the oil cooler or just the seals? 2012 TDV8 Vogue (L322)

A correction is required as I have misread the wrong bill. That was for the transmission cooler.

What I should have said is that the oil cooler and tank were replaced at 108,000 miles in December 2020, so nearer 50,000 miles ago.

As I will probably replace the doughnut intake hoses I will carry out an inspection in the 'V', or is it likely to be new seals required at this sort of mileage?

Hmmm. Any oil underneath the engine in the round cross member on the trailing edge of the engine undertray? Classic symptom. Gone to a good home: 2011 4.4 TDV8 Vogue SE Buckingham Blue with Ivory and clear glass = "Rory"
New Defender D300 90
1974 Series 3 Lightweight = "Millie"
My preferred specialist: www.glenrands.co.uk
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Thanks, I will take a look.

Finally I have it all connected and an engine start today. I ran it up for a bit and there are signs of warm air from the heater but I ran out of coolant so it's now cooling down. It never reached above a quarter on the gauge so I doubt the thermostat has opened.

I will revisit tomorrow.