iPhone apps not playing on Bluetooth Headphones

I have a set of Sony Bluetooth headphones which worked fine yesterday but today they won't play music via my iPhone.

My device connects successfully and I am able to make/receive calls using the iPhone but my music and other audio apps won't play on this device.

I have tried disconnecting the headphones and reconnecting, switching everything off and on, asking the iPhone to forget the device and have even reset my settings on the iPhone, but nothing has resolved the problem so far.

I see I'm not the first person with this issue and zero resolution, and would appreciate a solution.