Just after advice, I have been sent a Debt Collection letter

Thanks.
This is a private land fine. They are not parking fines whatever they may look like. I realise that many of them mimic those issued by the council or the police but they do not have the same authority.
A private land firm is not an emanation of the state. It does not have the right to punish you for the manner of your parking any more than your neighbours do.
This is essentially an invoice arising from a civil dispute arising from an alleged agreement whereby you agree to park for a fee. They say that you accepted a contract by parking there and breached it by staying too long or otherwise parking at variance with the contract. That is what is in dispute.
Private land fines used to be very easy to escape by refusing the details of the driver. They only have a contract with the driver. Unfortunately the Protection of Freedoms Act came into force in October 2012 and it makes the registered keeper liable for the penalty if he does not disclose the details of the driver within 28 days.
All that means is that the claim runs against the registered keeper. It just makes it easier to find the person against whom there should be enforcement.
The chances are high that they will not sue. Even though the law has recently changed in their favour and they were quite aggressive in the beginning, they do not sue in the majority of cases.
If they do sue then they would only have a claim for the sum of the original fine plus about £35 in costs. They will send you debt collection letters in which they will threaten to sue you for their debt collection costs. Do not be intimidated by that.
Even if they do go to court, they would still have to prove that the signs are adequate and clear. Quite often they are not.
They will send you some very nasty letters though. You will get debt collection letters making threats of legal action. They will probably get Graham Whyte solicitors to write to you as well. None of these are anything to worry about. It will not impact upon your credit history and it will not add to the costs.
Unless you actually get a court summons, none of these correspondences have any legal basis. If you do get a court summons then you can always part admit the claim and offer a small amount for their loss.
Hope this helps. Please let me know if you need more information.