I see where I'm going wrong now, Ulco.
I'd been leaving the values at defaults and setting only the second 'vector a' at '1', when that's the case, you get the result of an exact 45 degrees when using these settings...
Vector A = 1,
1,0
Vector B = 0,0,1
That's just changing one value from the defaults(and the reverse of this does the same but backwards, obviously).
Adding increments up to '90' to the middle 'vector a' parameter rotates this from 45 degrees through 90 degrees but, only when the first value is left at '1'. This is kind of confusing.
Zeroing these is the answer, so that only one parameter is applying rotation. Thank you.
The rotation values are quite tricksy on a plane object, you can stretch and skew them, aswell, along the X and Z's, this takes them off of square.
