Bit of a breakthrough with this - decided to check the bulbs in the gauge cluster. Fitted some new ones, and the front fogs symbol on the cluster is now lighting up. The rear fogs symbol isn't lighting up, but reversed the car up against the garage and could see they're working. Tried two new bulbs for that and no joy - but not a major concern to be honest. So looks like the fogs issue was caused by two sets of bulbs being gone.

With that out of the way - its more likely to be one of the more common causes. Diff sensor probably next item to check.