The first mechanic removed the rocker cover before speaking to me. I thought it was necessary to make a diagnosis?
After he'd removed it, he then came and knocked on my door telling me that it was a cracked head. I'm pretty certain he told me that he could see a faint crack, but I'm now hoping he'd actually used some type of test to decide this. That was when he said it would cost approx. £1000, but he wouldn't know until he could source parts. At no point did he mention the snapped bolts.
My second mechanic said that, judging by the amount of rust there was, it was likely to be a core plug problem. His issue with the bolts was that he wouldn't be able to get a proper seal with the rocker if they weren't removed. And he's not convinced that they can be. He was going to contact his engineer to see what he could manage.