Either you were parked on the line and correctly ticketed, or you were not and this photograph is misleading due to a parallax error (I can see only the single thumbnail in your first post). Whichever it is, they have failed to produce categorical evidence of your guilt.
I would contest the fine on these grounds, and ask for a clear photograph from behind the car or other appropriate line of sight in which they can unequivocally demonstrate the position of the rear wheel with respect to the line.
It might be worth checking the highway code for regulations / guidelines about parking in bus stops - a sign may not be necessary if there are clear regulations set out therein.
Similarly, whilst I suspect that the regulations refer to the position of the vehicle wheel, I would acertain whether the rear bumper oversailing the bus stop is sufficient to be in breach of the parking restrictions.
I usually have my parking tickets reversed, but I do take a perverse pleasure in hounding the poor bureaucrats until they give up...