But since I said 'the disk is MBR' I clearly don't have a GPT drive.If the disk is GPT you would want an EFI boot partiton and not MBR
I fixed it in the end by converting my disk to GPT and writing a new EFI partition, which in the end means I can boot from UEFI which is nice.The boot manager entry should not point to a logical partition for the the partition parameter. It should point to a volume designator.
If the boot partition is the first volume on the disk it would be partition=\Device\HarddiskVolume1