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what's my best course of action?
Don't buy a bolt extractor. Trying to drill the pilot for a bolt extractor is most likely to run the remnants into the fork, assuming a tapped-through boss, and it will either fall through and rattle around in the fork for the rest of your days or it will bottom out against the opposite wall. Since it's not likely to be seriously stuck, a left-handed drill is probably going to be the easiest way to get it out. 3mm or ⅛" should be about right.
I've just sheared a 5mm bolt in the front fork of my Surly Straggler.
I'm going to Screwfix tomorrow to buy a bolt extractor set, but I'm worried 5mm is too small.
I'm in East London - what's my best course of action?