You cannot use a spanner on the GA nipples as they are so low profile there is no room for the spanner flats. You need a 4mm or 7/16 allen key to remove it which is inserted into the nipple hole, and they are threadlocked in so very tight to remove, plus I think they are made of aluminium as our Magnacaps don't stick to them so if you use the wrong sized allen key it'll be easy to damage the nipple as its soft metal so be careful.
Heres a picture of the GA nipple on the right and a standard fill nipple on the left so you can see the difference. The 5 pence in the middle gives you a good idea of how low profile the nipple is.
http://www.demonpaintball.co.uk/pix/ga_nipple.jpg
I'll be taking a look at it for you at the weekend. When you spoke to Dan earlier he was thinking the nipple had a standard pin in like the one on the left of the picture which can bend under high pressure, the bend may be 100ths of an inch and not obvious to the naked eye, but enough to prevent a good seal. The pin on the GA nipple is very unlikely to be bent, I would say its most likely a slight bit of dirt in the nipple shaft or a damaged o-ring. Either way, i'll get you back up and running Saturday.
Matzy (Demon Paintball)