SMC reset did the job.
Thanks for instructions, I ordered the triwing screw driver especially to shoe in my Tb SSD.
This allowed me to disconnect the battery, there were a couple of minor reboot issues, but my main problem is that my battery has stopped charging. The little green light stays on, whilst the battery although acknowledging it is connected isn't charging.
Any tips? People have talked about SMC, which will be first move, followed by reconnecting the battery cable.
Thanks though for the guide a big help, onwards and upwards!