What CAJI says is correct, except that they can send someone to see you without a Court order. However, if they do the person who comes has no authority whatsoever and has no right to harass or bully you. They will not be a bailiff, although they might try to pretend that they are! Bailiffs can only come after a Court has granted a County Court Judgement. If anyone comes do not let them into the house.
I understand some debt collection businesses - including Capquest - sometimes buy debts where the debtor has disappeared. They then try to trace the debtor and if they find someone with the same name (or a sufficiently similar one) contact them alleging that they owe the money. So it is quite possible that this is not your debt at all.
Whatever you do, do not get involved in telephone contact with them - if they get your number and ring you, refuse to talk to them on the phone. If necessary, leave them to listen to loud music from your radio - placed just beside the phone you left off the hook!
Any letters should go by recorded delivery and keep a copy. As already said, demand proof of the debt.