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 postal order to pay it, and as people have said in other threads it's important that you don't sign your name on any correspondence with the debt collectors as some have been known to lift the signature. Once you have the CCA there are people here who will be able to tell you if it's enforceable or not.
Finding out when the last payment was made and how much will take a SAR letter which has a fee of £10 and the template for which is here, you may have to send it to the RBS depending on whether the debt collector bought the debt or is acting for them.
I'm pretty new to this as well and in the process of dealing with a debt collector myself so you might want to get confirmation before sending anything off, but the main thing is not to panic because as far as I can see there isn't much they can actually do without a court judgement beyond letter and telephone harassment, well aside from messing with your credit rating.