I bought a USB Bluetooth 3.5mm Stereo Audio Music Receiver Adapter from ebay
(I can't post url's yet. search for "USB Bluetooth 3.5mm Stereo Audio Music Receiver Adapter for PC Speaker Phones UK" on ebay the one I got was £4.39 + £1.99 postage, arrived in a couple of days)
It looks like a USB flash drive and you plug it into the B&M usb port then have the 3.5mm audio wire plugged into the aux socket next to it.
Then you pair it to your phone, and use your phone's music player to play music.
It means you iphone will be connected to B&M for the phone calls and a second connection to the bluetooth audio adapter for music.
Also, you will only be able to control the music from the iphone, not the steering wheel.
I only received it 2 days ago, so not used it much. Does any one know how to prompt siri from the steering wheel?
Hopes this helps