Welp.
Reflashed it with the stock kernel. Powertop shows the same usage of 100% on core. Also I'm now having problems with the touchscreen, it will randomly stop working and it can sometimes be for seconds or minutes. When it works its fine and as accurate as ever. So I'm guessing that somewhere there's a hardware fault which is a real shame as I've loved having this phone and I don't think there's anything even remotely like it out there.
Thanks for all your help guys but it seems like this was probably a lost cause.

tldr; phone borked, you're all great for trying.