Thanks all for the feedback here. I've ordered an NTSC version of the original chip from the US, so as soon as I get it I'll hook it up (assuming it works!) and set up mine to have the NTSC option pin so that it can switch between PAL (AY-3-8500) and NTSC (AY-3-8500-1) modes.
Probably take a week or so to get here (UK), but will update you (and the web page) once done.
Have fun.
Grant