Ive amended my original photo of the speaker block/connector. Am I right in thinking this is the correct positioning of the phone mute? Thats assuming its the right one Ive highlighted, from and too?
I think your right John, the RCD300 has the same wiring diagram. Had a quick google image search and seems to be right.
EDIT
Having been to Halfrauds today I can buy an ISO adapter but for 21£ to solve this problem but the nice young man was adamant I had to splice into one of the wires coming from the heater control panel, which I wont be doing. Anyhoo's after a cuppa tea and some more poking around, It looks like i need to swap out the yellow mute wire into the neighbouring connector block, which is smaller than where it currently sits. Maplin do a wiring repair kit, so we will shall see how that goes
EDIT
No luck. It was a household electrical repair wiring kit.