I'm going to change the oil on my TD6. On doing a 'search' and a 'wiki' I come up with both a 0w40 and a 5w40 anyone know which is correct?

The reason I'm trying a change, I have what sounds like a sticking tappet.. I'm hoping an oil change will help.

My FFRR handbook says:

Engine oil - Diesel vehicles

Use a 5W/30, 5W/40, 10W/30 or 10W/40 oil meeting specification ACEA: A3 and B3, and having a
viscosity band recommended for the temperature range of your locality.

Engine oil temperature ranges

5W/30 will protect from -30oC to 35oC (-22oF to 95oF).
5W/40 will protect from -30oC to 50oC (-22oF to 95oF).
10W/30 will protect from -10oC to 35oC (14oF to 95oF).
10W/40 will protect from -10oC to 50oC (14oF to 95oF).

Hope this helps.

Use the thinner 0w/40 then

What's the disadvantage of using 'thinner' ^^^

The advantage for you is if you have a sticking tappet, then hopefully the thinner oil will get into it and free it off.

I can see the advantage, just wondered if there was a disadvantage ?

Not in our climate

Will thinner oil not get past all the seals, scrapper rings etc and burn off quicker? Also, if the engine has done quite a few miles it will be worn and benefit from a slightly less viscose oil. IMO.. Previously..
Vogue SE TD6
Defender 90 2.4
Defender 110 TD5
Vogue 3.5 EFI

You need to read this then...

http://www.fullfatrr.com/forum/topic746.html Discovery 4 HSE
1998 110 TUM HS FFR Hard Top XD WOLF
1982 Series 3 Hard Top

*Gone:L462 D5 HSE LUX, L663 Defender 110 HSE, Discovery 3 HSE, 2014MY Range Rover Sport 5.0 Supercharged AB Dynamic; L405 Exec Vogue SE 4.4, 5.0 Supercharged Autobiography, Defender TDCi XS CSW, Defender TD5 HT, Vogue SE TDV8, Vogue TD6, RRSport SC 4.2V8, Classic 3.9 Vogue Auto, Land Rover Series 3 SWB

Modern engines don't suffer leaks the same as their older counterparts did. I wouldn't expect 0w/40 to leak out anywhere. As to the viscosity on older engines, it would need to be well worn already to do any harm