My light has turned up this morning, and the battery pack has a pop studded belt loop fixing. Just wondering what is the best method/place to mount the battery pack?

The standard pack is a bit shabby. I've got a spare L&M wide velcro battery strap that I put round mine, they're a few quid off CRC

One of these
http://www.chainreactioncycles.com/Models.aspx?ModelID=13176

Get friendly with someone with access to server racks – the velcro that comes with most cable arms is top notch.

I'll try and dig some out later.

Main issue I've had is the light vibrating around the handlebars but I didn't degrease the bar and clamp or take any care in installing it (whacked it on the commuter and away I went) and I have rigid forks, probably the biggest factor.

mount it where-ever you've space, the lead has loads of length. I've fitted mine under my stem or on the top tube of the frame without issues.. your choice.
As per Peter I've got some velcro strapping wrapped round mine to make it properly secure (as per BN mine is also off some server racking), though your "pop studded belt loop fixing" sounds new and may not need the extra strap.