Changing bank accounts

I have recently remortgaged one of my buy-to-let properties. Before it completed, but after I’d had the mortgage offer, I changed bank accounts. I rang them. They sent me a new Direct Debit mandate which I duly filled out and sent back.
When the mortgage completed, they sent me a summary of what would happen. They still had my old bank details. I phone today to try t find out what has gone wrong.
“Oh, we can’t use that bank,” said the assistant.
“Why not?” I ask. “It’s a normal bank, with a proper sort code and a proper account number. They just don’t make so many charges as the big boys, and they actually pay me a fair bit of interest.”
“Oh. I’ll ask.”
She comes back.
“You have to phone this other number, and they’ll be able to take your new details over the phone. You need option 3.”
Now I know better. Most banks insist on a paper version of the Direct Debit mandate. The problem is not that I haven’t given them information, but rather that they have lost or failed to process the information I have given them.
I phone and chose option 3. I seem to be put through to the department that deals with people in arrears. Come on now, I can’t be in arrears yet. I’ve only had this mortgage a month. They haven’t even taken the first proper payment.
I choose option 3. It rings and rings and rings. They do tell me they’re busy. Well, am I surprised?
I put the phone on speaker, but somehow manage to cut the connection. I redial.
“We are experiencing an unusually high number of calls,” a pre-recorded voice informs me. “Please try later.”
I’m still not surprised.
Has anyone had a straight forward conversation with a mortgage company about a change of bank account? If you have, please let me know your secret. I’ve had eight similar conversations with mortgage providers recenlty. Funnily enough, I own eight buy-to-let properties.