jjggh

Thankyou for your reply. My brother is a mechanic and he said the same. I booked in for it to be looked at. Was told to expect it to be not cheap. 
While waiting for the day, I decided to buy a new battery myself and try it. And it works. And the spare! I'm assuming the battery was almost dead and the one in my spare was dead so when I swapped them over that's why it worked intermittently. 
I'm cross with the guy in the key place though for telling me it wasn't the battery. He could have cost me a fortune  😫