Intrum Justitia are a debt purchase and debt collection agency that buy and collect other companies debts, which is why you may not recognise them.
Intrum Justitia basically bulk buy "bad debts" off finance companies like Capital One, Orange and Shop Direct. They buy the debt for a few pence in the pound and then chase the debtor for the full amount. This was if they get 20% of the people they chase to pay, they have doubled their money.
Debt collectors like Intrum Justitia tend to send a variety of standard letters, initially demanding you contact them by phone to arrange payment, then writing to you threatening to visit your home. They will then write again offering an out of court settlement, then threatening to take you to court to obtain a county court judgement or make you bankrupt. These are standard, system driven letters, so don't take them personally, but they can persist for months.
If you owe the money, then the best course of action is to repay the debt, if you are in a financial position do so. If you are unable to afford to repay the debt, or if doing so would stop you from being able to take care of your more important expenses, such as your rent or mortgage, then you should endeavour to make some arrangement with Intrum Justitia Debt Collectors to repay it, you may even want to offer to pay them a partial repayment to clear off the debt.