As well as sending 1 copy via recorded delivery, send 2 more copies with just a certificate of posting.
They could decide to refuse to sign for the recorded delivery one, but there is a case between a landlord and tennents, where the tennents refused to sign for a delivery, but having 2 additional copies sent with certificate of posting, the judge ruled in the landlords favour due to it being highly unlikely they did not receive notification.
It will be useful should the need arise to take it to small claims court.