I made one just like that about 15yrs ago for my F2 and its still doing the biz every now and again even today
I just wish I'd given more thought to a bit of cooper grease as the bolts are seized in now
which is preventing me from adjusting it to suit smaller chainrings.....
When I broke the ball end of the allen wrench off in the bolt I decided it was now in there for life as was running a 28/38/48 chainring set up :facepalm:
Love this bike and best of British luck to yah in BOTM
The ball end of an allen key is considerably weaker than the standard end. Try tapping the bolts with a lump hammer and then using either the standard end or an allen socket and the bolts will probably come out fine.
In particular I would recommend Facom allen keys as they are extremely strong and accurately made.
+1, i've snapped a 12mm ball end off before now. The straight cut end took the bolt straight out. (had to half dismantle the corner of the car to get the snapped ball out, and get access to the bolt! Turned a half hour job into a 2 day job.......)