The speaker in my drivers door appears to be a bit intermittent.

if you turn the volume up the speakers suddenly joins in and stays on but if you don’t turn it up it’s as quiet as a mouse.

has anyone else had a similar problem?

Not in an Octy but in general speakers that only work when you turn them up are suffering from cone or coil problems = dying. Not repairable, unfortunately.

agree with @StickyMicky. If it is volume related it is not likely a loose wiring connection but more likely internal to speaker itself.  Replace. 

I understand it’s difficult/ impossible to replace them as they are moulded into the door card unlike the bigger one at the bottom of the door.

Has anyone replaced them?

Sorry to interrupt, but exactly the same problem on mine. Hoping for answer from someone who sorted this.

I can only find pre facelift speakers at the moment