So used the balloon trick (put one over each end of the tank) to confirm the leak was definitely on inlet/gauge end of the tank. Used the video Rocko added to remove the end and immediate spotted something that didn't look right. Don't mean to be crude but it looked like someone had let a used condom in the tank
If you check the video above this is supposed to be the o-ring around the inside end of the air intake valve (large black o-ring in video). With as bad as this thing looked I'm surprised the air wasn't coming out of the tiny pin hole a lot faster.
Anyway replaced this o-ring as well as the one shown in the video which was hard as plastic and oddly shaped. Got it back together now and pumped to 200bar. Balloon has been on the end for about an hour so far and it has not inflated at all yet.
Thanks again for the information.