Acorn:
The letter is only asking who the driver was, failing to reply to that simple question is a offence.
Correct. Failure to notify can result in a rather large lightening of the wallet, so comply with that.
Was the vehicle in the area at the time where the alleged offence took place? The onus is on the owner of the gate to prove that, (A) it was and (B) it caused the damage. Without any video or photographic evidence, it will be nigh on impossible to prove.
However. I smell the “Well I have a witness” scenario here, and I would be informing my insurance company that someone is trying to defraud them.