I'm pretty sure the threat of bailiffs is just that, without a court order you have no obligation to admit anyone to your home anyway. Debt collectors love to make us panic and worry so we'll just roll over and do what they want, so personally I wouldn't worry until a letter from their solicitor arrives, as well as checking to make sure their solicitor is actually eligible to practice law in Scotland.
In the meantime, since as far as I can see the debt isn't statute barred, I think you're going to want to send off a CCA request letter, the template for which is here along with the £1 pos