Hi Royston,
You don't say what make of mirror you have aquired and there are dozens, however the following may be of some use . The vectra is a tricky beast to fit mirrors of any kind to because of the streamlined pod - I have Milenco mirrors which use two adjustable rubber protected clamps to secure the mirror to the housing, the mirror itself is clamped to a metal tube by two adjustable clamps to allow movement in and away from the door, There is also an optional rubber saftey strap which further secures the mirror towing mirror to the housing.
(this strap is very difficult to get right on a Vectra and is not essential to the operation, merely a "belt and braces" device)
Three important things I have found useful - fit the mirror clamps to the underneath straight part of the car mirror - watch out for the rubber protectors on the clamps falling off - make sure you set your car mirror before fitting the tow mirror as the mirror arm clamps foul the car glass and prevent adjustment.
Hope this helps and by the way once you have masterd the fitting they do actually work very well and can be fitted and removed in about 3 minutes