(Following up my own post above)
I went ahead and splashed out £1.20
on two '3' PAYG sims. These were not advertised as broadband ready and came subscribed to the default Flat 12 voice tariff.
Registered the phone number on the 'My3' website and topped up with £10 which after a few minutes was able to convert to the 'Mobile Broadband' addon.
Popped the sim in the modem and then after configuring the settings in the prefs panel, I was able to connect to a 3G data service.
Pete