They have to Legally inform Tenants of a rent rise. If they did not inform your daughter then the rise in rent is not legal and your daughter does not have to pay the money the Landlord says she 'owes'. She is only liable for the rise in rent from the day they informed her. If this Garage is rented on a more casual basis ie no written agreement then I'm pretty sure she still should have been informed at the time. How has it taken them 7 months to realise their 'mistake' if they were taking what would have been an incorrect amount from her every month by DD?