Munneyspinner - no matter how you try and justify it, you were wrong.
What you seem to be implying is that unless somebody points it out to you, and asks you to move your backside back to the seat that you paid for, its alright?
I take it you also think its ok to pay for the cheap seats at a concert and then go and sit in the front row if there is a spare seat?
Each class of seat comes at a price - fact.
You are obviously well aware of this, and choose to pay the lower price - fact.
You then dont see anything wrong in sitting in a seat which is sold at a higher price - theft/trespass - whatever, it is still wrong!
prat