I think the jack lead is associated with the radio.
A look at the manual on line suggests the jack lead has a yellow and green wire, which is backed up by your post. The yellow is a listener -ve and the green is listener +ve.
On the 400 / 500 series, the blue wire is the talker +ve so I think
green goes to blue and you simply ground the yellow wire.
I wouldn't bother earthing the shielding and certainly the manual makes no mention of it.
I'd sort of assumed having a name of gunnerstu and living in Salisbury, you would be either military or ex military and as a result I was happy to help but I've just had an unpleasant thought that you might be an Arsenal fan in which case I still hope the above works for you but that's out of sympathy rather than respect