1. A lot of this type of letter is a threat to try to get payment, but sometimes the threat does materialise.
2. They can't send bailiffs unless they first get a County Court Judgement, but they can do this by using the last known address - so the debtor doesn't necessarily know it has been done.
3. Keep the evidence the previous post suggested ready to hand to show to bailiffs if any do come, but do not let them in. They cannot come in by force on a first visit or push past you at the door, but could come in if a door or window is left open.
4. I know of a similar case, where the bailiff asked for identity and wanted to write down details of the house owners credit card! So don't show them anything with account numbers etc. There is no reason why they should write down your details anyway.
5. If anyone comes, make sure you see and note down their identity - including the company they work for.