"if i plug the aerial lead from the sky box to the main rf ('uhf/vhf') socket on the tv, i can then watch terrestrial tv on the av9000 but nothing on any of the link tv's ?"
Therefore you must have a lead from your aerial socket on wall to the sky box aerial in, lead from rf output 1 or 2 on sky box to the AV 9000
If you were getting Sky in other rooms with some configuration involving the av9000, then your link room tv,s must be tuned to the output frequency of your skybox, this assumes that the av 9000 is not on the same link frequency as the avant.
Why don't you try just plugging your aerial return feed into the other output of the sky box
Mark