Sorry for the confusion. OK I went to look at the PC and at first it was
like this.
It had both a USB keyboard and mouse. However, using a USB to PS/2 adaptor
on each, the keyboard and mouse were both plugged into the PS/2 ports on the
PC.

I tried different configurations. At first I tried unplugging the mouse from
the PS/2 port, removing the USB to PS/2 adaptor and then plugging the mouse
into a built in USB port. This made no difference.

I then tried removing the USB to PS/2 adaptor from the keyboard and plugging
this into another built in USB port on the PC (no USB hubs were used). This
also made no difference (therefore the keyboard still worked but the mouse
did not).

I also tried plugging the mouse into the PS/2 (using adaptor) and keyboard
into USB but nothing changed.

By the way I have tried a different mouse also and this has not helped.

Hope this makes more sense.

Kind regards

Dave