I had the same problem with mine last month, I knew from the first attempted turn of the bolt it was not looking good.
I ended up getting a replacement from a breakers, so at least knowing you can get one nearby for if/when it snaps would be good preperation should certain techniques not work.
Tried putting heat on it, molgrips, drilling it out, welding on a nut! Just cannot get the remaining stubs out.
New bolts went in with a bit of copperslip.