Thank you all so much for the help.
I too had the same issue as previous posts and I am happy to report this thread saved me an expensive visit to the dealer. True enough it was the fuel pump relay.
Fault codes read the same oxygen sensor issue and mis-firing.
I cleared them using an Veepeak OBD adapter and a free app.
I paid just under £20 for the relay and it took me just over an hour to fit, this included removing the glove box.
My aim now is to fire REX up at least once a week, to keep the relay from sticking.
I am a happy owner once again.