I've just found a set of green mateys from my old one, they look fine and work but don't seem to charge on the lcd when plugged in via usb.

Did I dream they were rechargable?

There's a little switch in the battery compartment which can be set to Alkaline or Rechargeable IIRC. If you've the set on the wrong setting the batteries wont recharge.

Another possibility is that the batteries are knackered. Some types if left discharged (empty) for a long time will refuse to take a charge.