I can't make out how the wrong sort code was used either, especially as it wasn't "wrong" when the payments were being collected. All I can think is that I would've (obviously ?) given the right sort code to the original company, they then went bust & another eventually took it over, then it was passed on to this lot quite recently, & that somewhere between all those moves it's somehow mutated.
The only correspondence I've had was their request for the original policy document to be sent to them. That said payout would be made direct to the account where payments were collected from, didn't seem I had a choice. A daily limit may be a fair point but they've never said that, just that CHAPS is their preferred method.
Your point about the mortgage interest is a good one too, though in my case the mortgage isn't actually due to be paid off 'til next year so doubt the building society would go with that. That'll all be factored into my complaint :)
I've come across this
http://forums.moneysavingexpert.com/showthread.php?t=2344179
which are the 2 sort codes but with a limit of £10 to 07-44-56 (the one the endowment company have) that would make sense. Still doesn't explain why the building society never received it to their holding account & returned it though.