That could well be dimistyle and thanks, it confirms my thoughts. I purchased the car with one working key (with the short slot - presumably the copy) and the other long slot bladed key in pieces. I then bought a cheap case on Ebay and transferred the electronics with a new battery and the 'long slot' blade to that. That's now the key which seems to work best.I also suspect the cheap key RFID chip is not recognised each time and the original works as designed.
We have been using the original non button keys for months as original flip key buttons no longer function.
I have bought new case and internals to get the remote locking working once again but made sure the 2 original keys have stayed programmed as back up for this reason.
Just thinking about it, we kept the broken key near or in the car until I repaired it and then kept it separate from the car. I now think that's when the starting trouble began- perhaps the original key is the only one which communicates with the immobiliser.
How am I going with this conjecture?
I rang a dealer and they said that it's likely that the locally cut key will fail at times especially if the original key is nowhere near the car. He mentioned that the local key would probably work if the original is in range. They can 'tune it in' apparently.