After trying everything above, mine turned out to be one of the little spring clips for the battery (the smaller, removable one).
I worked this out after buying a 2nd fob, and swapping the circuit board over to find it still didn't work (although I was still able to pair the new key with the old circuit board using the reprogramming method), meaning it had to be the battery side of the original fob.
It was the smaller of the 2 little spring clips where the battery sits.
Obviously the top of the springs clips are for making contact with the circuit board, but you need to ensure the bottom of the clip will touch the base edge of the battery when fitted.
Working 100% now.
I worked this out after buying a 2nd fob, and swapping the circuit board over to find it still didn't work (although I was still able to pair the new key with the old circuit board using the reprogramming method), meaning it had to be the battery side of the original fob.
It was the smaller of the 2 little spring clips where the battery sits.
Obviously the top of the springs clips are for making contact with the circuit board, but you need to ensure the bottom of the clip will touch the base edge of the battery when fitted.
Working 100% now.