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Automatic centre punch
https://www.screwfix.com/p/centre-punch-13-x-130mm/4338v
from Screwfix.
Punch an indentation, axially, close to the circumference of what remains of the bolt.
Punch it several times to make the indentation deep.
Once you've done that, place the punch tangentially in the in the indentation and punch tangentially so as to rotate the bolt out.
I've successfully removed a sheared bolt in a clamp at the top of a seat post using this method.
@mdcc_tester & @Constable_Savage thank you both for the replies.
Do you have any idea where in London I can pick up a LHT drill bit? A google search returns nothing and I'm meant to be riding this bike to France on Friday.