Airlines do not have either the time nor the resources to go chasing relatively small amounts.
Wrong, I'm afraid. They have robotics which do precisely that and which look for patterns where revenue leaks can be prevented.
You may get away with it for a while, or indefinitely (I hope for your sake you do, I have no axe to grind with you) but I can assure you that individuals and agencies do get debited for exactly this.
Whether or not such a charge would stand up in a court of law if challenged I don't know, I believe there have been precedents in the US but I don't know the outcome.
In this case, the difference could be about £400. Airlines are looking for every possible opportunity to maximise revenue.