Scult ergonomic keyboard numberpad stopped responding

Hi,

I have a new microsoft sculpt ergonomic keyboard. It comes with a separate number pad, which after working perfectly in the current set-up, suddenly ceased working. No input from the numberpad is received by the computer (numbers, num lock, calculator, delete etc).

I have a Dell U2312HM VGA screen with USB adaptors, which I have the keyboard and mouse USB receivers plugged into.

The USB ports work with other devices.

The keyboard itself still works, just not the numberpad.

The numberpad still doesn't work if I have the USB receiver plugged directly into the laptop.

The drivers are up to date (Apr 2013)

This set-up worked perfectly for the first month. Unfortunately I didn't notice exactly when the numberpad ceased working, so I don't know if something specific triggered it, but the set-up has not been changed in any obvious way.

Laptop: Acer Aspire 5742, Intel i3 CPU, M370@2.4GHz, 4GB RAM, 64 bit OS

Running: Windows 7 Home Premium, SP 1.

The most obvious thing is a battery issue - but it's a special flat battery, clearly not designed to be changed often, and the keyboard is less than 3 months old, I would be surprised if the battery had run down already. The AAA batteries in the keyboard section are still working fine!

Any suggestions very gratefully received.