I Blew it – rather big time – 200 miles and three ferries from home.

[Lucky]

Ahem...  :D :D

i suspect that now that the pressure is off the threads of the bolt (because it's broken), & you remove the rest of the valve cover, you may be able to either just grab the threads with pliers if any threads still stick out & take the broken remains out, or carefully turn the threads out with a pin punch or small chisel & light hammer.  a "reverse" or left handed drill bit would be the easy ticket if you have one available.

[h2olawyer]

I thought you had mentioned the reverse bit, Lucky.   ;D  Just figured I'd add where they tend to be available - at least in the US.  May not be the same in Norway.

H₂O

[Night Vision]

Ahem...  :D :D

i suspect that now that the pressure is off the threads of the bolt (because it's broken), & you remove the rest of the valve cover, you may be able to either just grab the threads with pliers if any threads still stick out & take the broken remains out, or carefully turn the threads out with a pin punch or small chisel & light hammer.  a "reverse" or left handed drill bit would be the easy ticket if you have one available.

give that man a donut

[Lucky]

Mmmm...dounuts....guguuguugu....+