Just got another email from them...
Firstly they acknowledge that the case was escalated to debt recovery incorrectly and I can ignore those letters.
However, they also state that
As per the Single Code of Practice, Annex C, the driver is often the same person as the keeper and/or the hirer. Where a keeper or hirer fails or refuses to provide the name and serviceable address of the driver when requested to, it may be assumed they are the driver, based on that failure or refusal. In absence of driver details, you as the registered keeper can therefore be assumed to be the driver.
While you are not obliged to provide the driver’s details, in the absence of that information we are entitled to pursue the registered keeper for the outstanding balance in accordance with the applicable legislation. Our request for driver details was made to allow us to transfer liability where appropriate.
In the meantime, the Parking Charge remains valid and payable.
So, they say the registered keeper is liable in the absence of the driver being identified and still want paying.
Any advice on how to respond would be appreciated.