A property licence does not transfer when a property changes hands. If you buy a let property that needs a licence under Northumberland's rules, the existing licence ends and you need to apply for a new one in your own name. The seller's solicitor should disclose any existing licence and any open enforcement notices.
For conveyancing in Northumberland, ask three things before exchange. First, is the property inside any current selective or additional HMO scheme area. Second, is there an active licence in the seller's name and on what conditions. Third, has the council issued a civil penalty, banning order or rent repayment order against the seller in the last six years.
Owner-occupiers buying to live in the property do not need a licence. The rules only apply when a property is rented out.