Please post the letter you got from Gladstones - is this a Letter of Claim?
Did you receive the original PCN and reminder - and are they addressed to your current address? Are you the registered keeper - car not leased or something? If you are reg keeper and this is the first you have heard of it you may be able to complain, which they will have to treat as an appeal. They will reject it of course, but it puts them on notice that you intend to defend it, not roll over and cough up. Their PCN is not POFA compliant so they can't transfer liability to the keeper anyway - hopefully you haven't identified the driver (even in passing by saying I parked or similar) in contact with Gladstones?
It's likely that Gladstones will move it to a court claim, but they will make a mess of the claim and you will be able to defend it with advice from here.