I've traced this to where the coolant level sensor fits into the base of the expansion tank (M62 V8 engine). I have't taken it out yet, but is there a simple to replace o ring seal there or do you have to replace the whole sensor? Thanks. Drives:
2010 FFRR TdV8 Vogue - Stornoway Grey
2010 FFRR TdV8 Vogue SE - Zermatt Silver
1998 BMW E36 M3 GTII

No part listed for o ring so must come with sensor .they aren’t expensive tbh .

Click image to enlarge

https://www.island-4x4.co.uk/sensor-coolan...31160.html

Blimey, no they're not expensive are they! Just assumed they'd be silly price. I'll get one and see if the leak stops.

Thanks all. Drives:
2010 FFRR TdV8 Vogue - Stornoway Grey
2010 FFRR TdV8 Vogue SE - Zermatt Silver
1998 BMW E36 M3 GTII

There is no O ring for the sensor as there is no fluid at that join. The sensor is a magnetic reed switch and can be removed with the tank full. The tank splitting at the seam is the most likely culprit 2007 TDV8 VSE
2003 TD6 gone.
2002 P38a gone
1999 P38a gone
1997 p38a gone
1993 VSE gone
1992 VSE gone
1966 Series 2a with V8 conversion gone

Hmm, that's interesting. There is definitely fluid dripping from the bottom of the level sensor. Maybe it has sprung a leak in the header tank although this one isn't that old (the old one failed spectacularly!). I'll see how it goes.

Thanks again. Drives:
2010 FFRR TdV8 Vogue - Stornoway Grey
2010 FFRR TdV8 Vogue SE - Zermatt Silver
1998 BMW E36 M3 GTII