Yes I've already tried that but no change.

try clear CMOs settings.  To reset the CMOS and clear those settings:

1. Turn off the computer and disconnect the power cable from the electrical outlet.

2. Remove the computer cover.

3. Locate the 2-pin password connector (PSWD) on the system board.

4. Remove the 2-pin jumper plug.

5. Locate the 2-pin CMOS jumper (RTCRST) on the system board.

6. Move the 2-pin jumper plug from the password jumper to pins into the CMOS jumper.

7. Plug in AC power to the system and wait ten seconds for the CMOS to clear.

8. Move the 2-pin jumper plug back to the password jumper.

Okay. Just fitted a brand new CMOS battery but still the same!

Re: 2-pin jumper between pins 2+3 in the RTCRST connector

move this jumper plug to pin 1+2 of the RTCRST jumper, wait 10 sec, move it back to 2+3.

no need to touch the password jumper plug.

Thanks for your reply but I'm somewhat confused about those instructions.

Before making any changes I have a 2-pin jumper between pins 1+2 in the PWD connector and another 2-pin jumper between pins 2+3 in the RTCRST connector. These, I assume, would have been factory-fitted.

Point #6 doesn't advise which two pins should be linked on the RTCRST connector?

Can you advise further and thanks for your help.

Okay I've done that but still no change.

I'm beginning to wonder could the problem possibly be related to the fact that I'm connecting the Keyboards (I've tried a second one) via a USB dongle as the machine does not have a PS/2 connector on board?

Or could it be the dongle that is faulty?