Thanks Johnny and Bubbles. And thanks to everyone who who answered my questions / gave support.
My assessment was on the 28th October and I got a letter quite soon after that saying I had scored no points. I'm not sure exactly when the DWP received the GP letter but it was within the mandatory reconsideration period. They wrote to me about 10 days ago saying they were sorry it had taken longer to respond than anticipated but they were now satisfied that they had all the relevant reports etc and would send me a decision soon.
In between that I'd sent them a data request and discovered that they already had a GP letter from December 2014 for my ESA which listed various problems I had as a result of my brain injury and there were reports from THREE other Atos health practitioners all dated December and January saying that they agreed it was unlikely I'd ever not have a mental impairment. Now the report that was written for my PIP said I had no mental impairment. I didn't get the chance to send this in to the DWP but I reckon they possibly joined the dots and realised that they had made a mistake not cross referencing their paperwork.
I recommend that anyone who has been on disability benefits long term actually find out what info the DWP has on them, phone records the lot. If your'e receiving ESA because of a mental health problem then some of the descriptors might cross over too.