Fitting my Nur spec today and i noticed a touch of oil around the rear diff looks like its been leaking a while alot of sludge around the area, where isit coming from

 

Would it be too obvious to say... the rear diff?

It's not uncommon to get a tiny bit forced past the shaft seals, and you only need a tiny bit to result in muck picking up around the casing so it may look worse than it actually is. Beyond the shafts there's only the backplate gasket, breather (up the top of the backplate), input shaft seal and the filler & drain plugs. Only way to find out for sure is to get underneath and have a look, checking the oil level while you're there*.

*=obviously you need the car to be level, preferably on the ground, rather than jacked up at the back only, when you check the oil level!