Finally!

At last I've traced the source of the squeak, it has been driving me mad for the last week. 

All of a sudden, out of the blue, the Brooks started squeaking and could I trace the source of the squeak, could I heck. And with such a normally rattle free and silent bike it was infuriating. 

However, today I traced it. It was solved by some light oil (GT85) at the base of both springs where they bolt into the saddle frame. Silence is now very much golden. Peace at last!

I love the wonky Brooks plaque on my example, it highlights to me that this is a thing made by real people and all the better for it.