Thanks for all the ideas, sorry I've been slow to reply.
I can report the fox is still at large and hasn't breached the coop. My run is fully netter and has solid sheets of corrugated iron around the bottom so no chewing or digging through that. It could chew through the walls higher up though possibly. I had been planing to start free ranging the chickens but looks like that is out for now unless I'm supervising.
I can also report I believe the fox probably had the life scared out of it night before last...
I let my dogs out, and old Lab and a young Retriever x for a late night boundary marking session. They headed off around the run and all of a sudden it sounds like the hounds of hell are running loose! Grab the torch and run down to find them still baying, looking like warthogs with fur up nose to tail tip and letting the entire area know they were there. I was surprised the young one hadn't gone straight over the fence to continue the chase, though I'm kinda happy my boundary training worked, getting him back could have been a challenge.
Since then I haven't found any tracks inside the boundary which makes me very happy.
I will still run the hot wire as insurance and I would like to get a guardian dog before I start free ranging them. Unfortunately my dogs are not trustworthy with the chickens unless I an out supervising. My ponies however do a good job of running off strange dogs so I think the fox would get a bad reception out there.