Finally persuaded it to come apart, very surprised to find there's no washer, "O" ring or anything. The core is slightly conical and its purely the interference fit once the nut at the top is tightened that stops the water getting through. Solution, grease it before refitting, liberal amounts of grease then clean away the excess once it was all back together. Its drip free at the moment but the water is only just above tap level, see what happens when there's a decent head of water in it.
I'm no further forward as to what the screw in the front is all about, there are two holes in the core, the one that opens the tap and a second one that allows water through the hole when the screw on the front is removed
