@dc42 Yep, a ground wire and heated bed thermisters were both pinched and shorted together. Looks like I did a bad job putting things together.
Electricians tape on ground and thermisters resolved the issue of the hotend reporting incorrect temperature but the hotbed is still showing 2000 and the Z-Tower stepper's LED isn't lit so it looks like I'm not out of the woods yet.
I suspect power isn't flowing to the z-tower which is also connected to the hotbed and had the pinched wires.
Will work with SeeMeCNC support and hopefully figure this out.
Thanks again for your help, good to know I probably won't blow up another Duet (fingers crossed).