Microsoft Surface Ergonomic Keyboard pairing issues
The issue was with the version of Bluetooth. The built-in version (2.0, I believe) wouldn't configure with the keyboard.

I've since bought a Bluetooth 4.0 Dongle (cost about £12) and I can now use the keyboard. It's a bit of a faff, had to download a Bluetooth developer tool from Apple and I now have to manually switch from the internal Bluetooth to the dongle before I can use the keyboard.

Thanks for all your suggestions, though, and hopefully anyone facing the same issue now has a solution.

Kieron