Hi Andy,
Interesting post - if unlikely to be directly relevant to many others here
I guess you have just fallen foul of a "rule" and will probably have to pay the higher rate in the end.
If that proves to be the case, you paying £750 and the charter company topping up the fees to the commercial rate would seem to be the fairest way of dealing with this.
As to what exact "rule" governs this, who can say? You may have highlighted a situation never encountered by the authorities before.
Good luck and let us know what happens.
P.S Don't take any "robust" replies to your post to heart - the boys can play pretty rough om this forum. I got slapped down on my first post, but lived to tell the tale