So, I should have taken the warm SD card as a sign, I got home and connected up my new Pi 2 only to be presented with the same symptoms as the previous board.

I put the SD card into a PC reader and got some strange results from Windows trying to read the card. I grab a spare Micro SD and throw a fresh copy of Noobs on it and sure enough the Pi boots. I swapped the card into the old Pi 2 and it leaps into life also. Score!!

So now I have two working Pi 2's, a cactus Sandisk 16Gb MicroSD and no working, tested OS left. On the bright side, I can now do project logs on the configuration of Raspian to act as the encrypted bridge.