Aim between the eyes...
.....i think you'll find that they operate a "virtual office" system. For my service pension I get letters from Ty Glas, for other stuff I've had letters from Leicester, Liverpool and Glasgow and spoken to chaps in Portsmouth and Liverpool. They all have access to the same data base and the notes made by the last person you spoke to. So far, they have all been helpful and eventually we've managed to sort it out amicably.
Make sure you keep notes of all phone calls, when, who you spoke to (both names!! it avoids the "Kevin?? which Kevin? conversations). Be patient polite and persistent and you'll get to the right answer.
The Ancient Mariner