Ok, it's fine, the letter is just a debt collector intimidation letter, and in my view, illegal intimidation, because there is a legal process to be followed which does not include debt collectors at this stage, so passing on the matter to debt collectors is unlawful. That's my view, anyway. You will notice that the letter makes no mention of the facility to submit a Statutory Declaration.
So, on to debt registration, which will add £10 to the amount, and you can then submit a Statutory Declaration to get the matter reverted back to the PCN stage.