The Best Buy is a 10ltr bottle at £14 however that doesn't come with the screw fitting required to ensure as little as possible is spilt either on the car or on your hands so I opted for the 4ltr bottle at £12........there is reason to my appeared madness

The 4ltr bottle has the screw fitting ensuring a proper fit when filling the reservoir so I've kept that and will in future buy the 10ltr and decant into the 4ltr bottle.
Simply open the bonnet, the current F15 doesn't have two tanks as it appears some of the E70's have and it's simply a case of unscrewing the blue cap in the engine bay, screw the provided spout onto the 4ltr bottle and then turn it upside down and screw the fitting onto the now revealed screw thread where the blue cap was.
With some gentle pressure downwards the bottle empties itself into the tank and hey-presto, I'm now nearly full again.
I had thought of taking the car to the dealer to refill it however it appears as ever they pull your pants down and it really is an easy 2 minute job. I didn't spill a drop however I did wash around the filler with a little water just in case as I understand it's pretty horrible stuff.
Anyway, some may have done this but if you're like me and the wrong side of 50 don't be concerned and save yourself some money.