Neil - TR have it mis-labelled. It is in fact an 8.7mm tube (actually a tad more) and it takes a parker style. I've told them about the error several times, but they don't seem to have caught on.
Dan - I have to agree with Neil's last point, and the previous comments about two piece 8mm parkers. There really is nothing descent out there at the moment apart from the retro which is a bit quirky. Where the Elegant American goes wrong in my opinion, is that it doesn't have a centre band - not just from a aesthetic point of view, but also from a practical one. If you get the length of your cap barrel wrong, even by a fraction of a millimetre, the mechanism sits wrong and it won't twist properly, meaning you have to spend half as long again as it took to make the bugger, in order to sort it out. Very rarely is it plain sailing to assemble them in my experience, but perhaps I'm approaching it the wrong way.
So yes, much as I would like to see good ES equivalent in the UK, I would also REALLY love to see a GOOD two piece G2 mainstream twist pen that isn't too chunky or top heavy or quirky, because as far as I'm aware, there just isn't one at the moment. Not much to ask is it?