TM 7 (filled out)
Marriage Certs (both originals)
Wife's Blue Book
Wife's ID card
Bank Letter
Map to house (photocopy)
Couple of family photos
Children's Birth Certs (if you have kids)
Doesn't look like there's anything difficult there that would need the actions of an agent. In fact you'd be just as quick doing your extension in person instead of taking the trouble to travel to an agents office. No resolving, it's all pretty straight forward.