Thanks Antiburn for the useful info.
After a bit of rooting around with the M30 engine info (can't believe I didn't know that
) I discovered that the recommended plugs are Bosch W8LCR or NGK ZGR5A or Champion 2415(Gold)/3145 (Platinum) plugs. It's the product numbers which were evading me for a while and causing my headache. It seems I can get plenty of those over here nice and cheap.
Thanks for the help.