Success at last. Managed to get the front fork caliper allen key bolt free.
What a rusty old mess.
I bent the mudguard back - but it is still 100% in-tact and can be put back later in the year, I removed the brake cable so I could spin the caliper around.
Once this was out I could start whacking. Took me a while to find the right combination of things to use next....
Used a large allen key inside the forks on the face of the bolt and tapped away. It gradually it broke free. It is now measured (30mm), replacement on order. Greased it up and re-installed.
Gave it all a proper clean out too.