APCOA are benign - we've never seen them litigate and if they did they'd very much struggle to enforce against the keeper. Ignore them. If you were to receive a Letter of Claim, come back for advice, but this is exceedingly unlikely.
Of more immediate concern is to see if there's a reason that a debt recovery letter is the first you're hearing of the matter. Get out your vehicle's V5C document and check it bears your current name and address.
