Gladstones have come back with this:
"Request for Erasure of Personal Data
We refer to your request regarding a request for erasure of data.
Unfortunately, we cannot process your request because we can demonstrate compelling
legitimate grounds for the processing of your data, which override the interests, rights and
freedoms of you as an individual.
That legitimate interest being that we are acting on behalf of our client Parkit Management
Limited for the recovery of monies relating to an unpaid Parking Charge(s), number(s)
To process your data is necessary and lawful, as it is required in furtherance of our client’s
instructions, and we act for them on their rights and obligations for the recovery of such
monies.
We note your request for previous addresses to be removed from the matter however, they
now form an integral part of the file given it is going through legal proceedings. Please be
assured we have now rectified your address and therefore no further correspondence will be
sent to the previous address as of today’s date.
If you are not satisfied with our decision, you have a right to complain to the Supervisory
Authority. At this stage you should contact the Information Commissioner’s Office (ICO);
Address; Wycliffe House
Water Lane
Wilmslow
SK9 5AF
Telephone Number 0303 123 1113
Please note that time limits may apply and so you should contact the ICO as soon as
possible following our decision.
We must also confirm that you also have the right to a judicial remedy (ie claim
compensation through the courts), if you remain unsatisfied with the outcome of a complaint
to the Supervisory Authority and you believe there has been a breach of the General Data
Protection Regulations 2018. The ICO cannot award compensation and so if you cannot
reach an agreement on the amount of compensation where appropriate, you can apply to
court. You should seek independent advice at any stage that you feel it necessary."
I suppose there is nothing to do but to wait for further action from them, as there is nothing I can do at this point.

Thanks for all the advice again thus far!!