Apple wireless keyboard - couldn't connect to the device

I have a Mac Mini (Late 2012, OS X 10.9.5) that I use with the white wireless key board and mouse. This has worked fine until two weeks ago when the keyboard did not connect to the Mini. I followed the support help and switched off and on the keyboard. The bluetooth preferences shows the keyboard but when I click 'pair' it fails with "couldn't connect to the device". I took the batteries out while I was away for a week but this had no affect. I have also tried using the preferences 'debug' settings to forget the devices and reset the bluetooth module. This also does not have any affect of the keyboard - it always fails to connect.

Is there any way to get the Mini to connect to the keyboard?

Best regards

Andrew W.