You will need to check the terms of the licence agreement as to any specific length (for example our lodger agreement requires a months notice if there’s no reason being provided) but just a letter will suffice. No prescribed notice is required. If there is no mention of a length of notice to be given, it must be “reasonable” in length. A month would normally always be reasonable and anything less would possibly be risky although it would depend on why (for example if they’re not paying, 2 weeks may be considered reasonable but it will be down to the court to decide).