Understood. based upon what you say, I would tend to agree with your conclusion on one or both of the following bases: -
1) any creditor claiming a debt must do sufficient evidence to establish that the party from whom they're claiming the debt is liable for the debt. From what you say, they have failed to do so to date.
2) even where the above can be shown, it may be from the dates you refer to, that the debt is time-barred under the Limitation act. even where you did owe the debt, of what you say, this is not the case, unless the creditor can demonstrate that you have either made a payment towards the debt or acknowledge the debt within the last six years, even where you would otherwise be liable under paragraph 1, the liability for the debt would be time-barred.
Accordingly, you may consider responding to the debt collectors advising that they have neither evidenced that you owe the debt in the first place and notwithstanding this, the debt would in any event be time-barred under the Limitation act for the above reasons.
You can say that if the debt collectors have further evidence to produce in respect of the above, that they should do so by post, but failing this,
you can ask the DCA to cease and desist further communications unless it is to notify you that they are issuing court proceedings. There is a great deal of legislation that can protect you which I suggest you refer to as follows:
Section 40 - Administration of Justice Act 1970.
The Consumer Protection from Unfair Trading Regulations 2008.
Section 4A - Public Order Act 1986.
Section 1 & 2 - Malicious Communications Act 1988.
The Protection from Harassment Act 1997 - section 1 can be applied to creditors harassing debtors whereby a person is guilty of this offence if they pursue a course of conduct that they know or ought to know amounts to harassment of another. You would need to prove their conduct on more than one occasion.
If they ignore you, you can advise them that you will otherwise consider a report to the Financial Ombudsman Service as to their conduct with a view to reviewing their licence and to seek compensation and also reserve your rights to seek civil injunctions and costs against them.