Possession and

Rent Arrears

My tenant's assured shorthold tenancy expires on the 27th February. As he is in rental arrears I wish to end the tenancy asap.
I understand I have to serve a section 21 (1) b notice giving two months notice but do I have to serve this notice myself? Since I live some distance away from the property it would be helpful if I could delegate this task to a third party. If delegation is acceptable can you recommend a reliable organisation that could serve notice legally and be acceptable to the Courts if the tenant refused to vacate?