Well, it appears I have a different problem.

With the use of a bigger spanner I have released the bolt, removed the fog light unit, and replaced the bulb.

Not only do I not have a working fog light, but now I've noticed that the sidelight isn't working either.

To be honest, not sure if the sidelight was working previously, or if it's something I've managed to disable during my fumblings.

Do they share a common harness somewhere that I might have dislodged? Any other suggestions?