
Chris, you are the man, So simple when explained, have just swaped all over and waiting on bypass box.
Below is the reply chris gave me which was like "turning on the light"

(me with the smiley face now)
You will need your black box however, as that and the wire its connected to (not the existing seat mat) will come off your old seat and go onto your new seat with the bypass box. So basically you will have three components, one will be the new bypass box, the other is the black box from under the old seat, and the third is the wire that goes from the black box to the black/yellow connector.
So remove the sliding cover from the black/yellow connector on old seat.
Remove the wire which goes to the black box from black/yellow connector.
Unplug black box from seat mat.
Then on new seat
Unplug seat mat (grey wire from photo) from black yellow connector.
Then plug in all three bits to the new seat, so bypass goes to black box, and black box to the black/yellow connector via the wire you removed