And another Bank Holiday bites the dust

So I eventually get me new light bar on and wired but while I'm down in the fusebox area I starts to poke at a patch of rust at the bottom of the A post. Bad idea. Big hole emerges. So I gets it away to my local spannerman for welding (his is bigger than mine and gets more penetration so he tells me - Ooo Errr)
Went to pick it up - wouldn't start - no starter, nuffink. Bonnet up, hit the battery terminals with a spanner and it's right as rain. Until I gets home. No handbrake. Clicky sounds from the lever but no action at the grippy end. Pulls the gaiter off the lever and discovers the cable has snapped just below the connecting end bit.
This just leaves 2 questions:
I know its possible for the cable to act as an earth lead when other earths are a bit croaky but is it possible to fuse through the actual cable? That might explain the iffy starting.
Is it a big job to replace?

Or the welding earthed through it?

So is a mig hefty enough to burn straight through a h/b cable?

it's not unheard of unfortunatley