Almost embarrassing writing this but I hope nobody else does it.
I took the battery home to give it a good charge and put it back in on Saturday, I missed one cable, probably a critical cable, the earth for the starter.
I could see the starter light stuck on and there prop didn't turn. Smoke started pouring out of the engine bay and of course all the switches I flicked off. No change, still heaps of smoke. I grabbed the fire extinguisher and thought I crossed a wire so my first aim was to disconnect the battery.
Succeeded in disconnecting the earth cables. What a mess. After talking to my mechanic, he has seen this happen before, the positive somehow jumps, not sure how and it looks like it used the throttle cable as the earth. The throttle sheath melted and the metal also melted the side of the battery.
A solution is to cable tie the cables so none of them can't drop out of sight. Been flying for 9 years and haven't done anything like this before.
Please take note.