Hi
Can anyone tell me what the ideal weight for a westie should be (adult) I have been told it should be around 16lbs, if so then mine is overweight

The KC standard doesn't specify a weight, just a height of approximately 11 inches.

Each dog really needs to be assessed on it's on frame.  If you gentle rub down the sides of the ribs you should be able to feel them easily but not see them clearly visibly.   Westies when the carry a bit too much weight tend to have a ripple at the base of the tail and at the neck line.