I can confirm that the advice given by morlock has returned my bios to working order, though I don't think this marks the bug as solved as it is just a workround and will probably recur if Ubuntu is reinstalled.
The bios contains no settings related to Uefi that I can find (ie I can't switch it off). And YannUbuntu, who kindly read my latest Boot-Repair pastebin report, tells me that Secure Boot is not enabled.
I can confirm that the advice given by morlock has returned my bios to working order, though I don't think this marks the bug as solved as it is just a workround and will probably recur if Ubuntu is reinstalled.
The bios contains no settings related to Uefi that I can find (ie I can't switch it off). And YannUbuntu, who kindly read my latest Boot-Repair pastebin report, tells me that Secure Boot is not enabled.