So i figured i'd whip off the 52T and put the 50T spare back on for now, sure enough the threaded bolt just fell out another came out with some grunt but the remaining two were firmly bonded, i tried my bolt removing bit which rounded off, then resorted to drilling them out using a normal HSS bit and whatever those screws are made from its very hard stuff, i barely made a dent, so off to screwfix for a specialist bit, after a little bit of smoke and effort out they came.
What i've learned from this
Throw away the Shimano bolts and find some normal aftermarket replacements
Shimano 5800 and most likely 6800 series cranks are very poorly engineered
Wear goggles and use a vice when attempting to drill something that has 52 sharp teeth
I need a bench drill
I need a set of new chainrings