I have been reading up on this. The TA comes with different bases for either the Vectra C, or Signum. I think (could be wrong) we would need the Signum base. Anyway, mine off ebay came with the Signum base and it's a damn good fit on my rear seat.
I don't know how the rear bit of the base attaches yet but there is a bracket required for the front of the base, which touches the transmission tunnel. From what I have read on another forum, this is part number 93182997 and when I called my dealer it was about £30. The bracket includes the ground and 12v power supply, which the connector on the underside of the base plugs in to.
There might be a way to 'bodge' installing the base but I would rather use the proper bracket.
Hopefully Jim will reply soon and let us know how he installed it, and then I will know whether to order the bracket. My dealer says it is a special order item.