Technically, assuming the car is safely loaded and complies with British and European Law, you shouldn't need any documents other than those you usually travel with, such as the registration documents and insurance for both the recovery vehicle and the broken down car. You may, though, attract the attentions of border staff, so the best course of action would be to get a letter on headed paper from your brother-in-law to show to customs staff along with confirmation of a specific appointment. You must also collect the car within 28 days to bring it back to the UK.