However, this presented me with a challenge, could I recover my phone without losing anything? Turns out the answer was yes
I was able to NVFlash (battery out, hold both vol keys and plug in USB) over a good recovery, and the phone booted straight to it, from there I restored the old boot.img that I had, rebooted and held my breath.
Success! OK, so wifi didn't work due to the old kernel, but whatever, quick CM reflash fixed that (in hindsight, I could probably have just done that straight away anyway).
I created a little batch menu system to handle the command and let you pick from an internal or external recovery (depending on what you normally use) and a couple scripts for Linux users. Hopefully anyone that has bust their phone to the point of NVFlashing will be able to do this instead and keep all their stuff