An LoC will tell you that it is a letter of claim or before claim r something on the lines. It will NOT be for the useless DRP as they are not a party to the contract allegedly breached by the driver.
A real LoC will come from wither the operator themselves arm, more likely, from a bulk litigator such as DCB Legal, BW Legal, Moorfields, Gladstones, et al. You will know it is a real LoC because they are obliged to give you 30 days to pay, not the usual 14 days that the bottom0dwelling debt collectors give.