Update. Whilst out in garage I wondered if a medium hollow wall fixing would work. Result. It is just long enough and wide enough to secure the battery retainer block.

Just had the same problem with mine today. Battery died with all this cold. Battery retainer nut and bolt rusted in place. Had to cut head off. Now off out to get a new nut and bolt for battery retainer. Thanks for info though, I was wondering how to get it out. Just joined this forum...