Just to feedback on this, I hacked a simple 5m RCA Phono lead in two (very thin cable) so had two RCA Phono ends and a bare end. Due to the size of the cable, I had to trip back around 6-7 inches of outer sheathing to reveal the earth wire. After lots of twisting and doubling back the copper, I eventually got a wire that I could connect the the speaker point. I then did the same for the inner signal cable but only stripped back around 5 inches of the inner sheathing.
Once done on both sides I could then connect one of the RCA ends to the Amp and then to the in wall speaker terminals, and the other to the sub and the opposing in wall speaker terminals, and et viola. I have a line level sub signal sent over speaker cable.
There is absolutely no hum when the Amp is switched on, but a good clean bass is heard. The only small issue is when the Amp is powered off completely (using a Standby Saver type gadget) then I do hear a small hum. Fortunately, the sub has an auto power off feature that puts it into a standby mode when no input signal has been received after 5 mins, which eliminate the hum. Still, when the Amp is on and working normally - all is good.