Hi Rich, thank you very much for such a fast reply.
However, the Num Lk button appears to work fine when pressed and unpressed - lights up and an audible sound to show its on / off - but then when I try to type in numbers from the keypad, there is absolutely no response, even with the Num Lk light lit up. The numbers on the top line of my main keyboard work fine (but are slow to use when keying in lots of numbers).
In summary, even when the Num Lk button indicator is lit up to show its on and functioning, when I press any number on the right hand side keypad it does not display on screen - cursor just blinks. This is the same with any programme I use; both Word and Excel etc.