Ok so after a bit of digging around I got to bottom of this one.

Obviously at some point the under bonnet fuse box lid has been incorrectly fitted. Water ingress into the fuse board causing problems.

I wasn't entirely convinced so rather than stick a new fuse board on, I was able to strip this one down by separating the 'sandwiches', clean and dry everything, refit = all good now.

Allow about 2 - 2 1/2 hours to strip, dry and clean, take a few digital/phone pictures before pulling all the pins out. If you like 3D jigsaws you will like this job.

I did have it sitting in the airing cupboard for 2 days (without telling the wife) but that didn't dry out out properly.