My first statement was just my interpretation, the second was a copy and paste.
A partial copy and paste - you missed out the second half, which is likely the one that is the basis for the charge. The reason for the charge is given as:
"101) FAILURE TO PURCHASE THE PARKING TARIFF FOR THE REGISTRATION MARK OF THE VEHICLE ON SITE AND/OR WITHIN THE TIME ALLOWED"
Regarding the first half, from what you have shown us, 1 hour's parking was paid for, and the vehicle was on site for 43 minutes - and I am working on the assumption that you have checked the ticket to make sure that the right vehicle registration was entered. If so, that moves us onto the second part (in bold above) - this would suggest that Excel have mandated some arbitrary time limit in which a ticket must be purchased after entering the car park. This should be specified on the signage, which is why it will be useful to get some photos of the signs.
A requirement to purchase a ticket in good time upon entry, and display it in the windscreen, makes sense in a car park patrolled on foot by wardens, as they need some mechanism by which to check that drivers have paid. However, when the car park is seemingly managed by ANPR cameras, there doesn't seem to be much legitimate reason for requiring a ticket to be purchased within a set time limit, as long as the driver pays the relevant tariff to cover the duration of their parking. The only purpose such a term would seem to serve is creating opportunities for it to be breached, thus generating £100 charges for Excel - an argument could therefore be made, as Andy suggests above, that the charge is not "commercially justified".
It's worth bearing in mind that such an argument is a good one, but is not one that Excel or IAS are likely to accept, and would probably be one you would need to argue in court. In the meantime, let's see photos of the signage to see what that throws up.