I've tried giving them a good whack and another heating as advised but still no luck so looks like I'll be removing the bolt heads by force.

Thanks for the words of reassurance gents! I'm always very nervous of doing something irreversible.

Waynovetten,

You're a star! Cheers for the offer but I might treat my self to some snazzy new stainless steel bolts now that I have the right size.

In my defence, I fitted the handguards with almost no issues... almost.