The entrance signs are quite prominent, and do draw the driver's attention to the fact there is a sign near the entrance that they should seek out. Although a lack of specific signage by the disabled bays could be argued as a breach of the BPA's code of practice.
But, you may have a technical appeal point to explore, as they don't seem to be seeking to hold you liable for the charge as the registered keeper (using Schedule 4 of the Protection of Freedoms Act 2012 - there's a link in my signature at the bottom of this post).
You could appeal, as the keeper, along the lines of the below:
Dear Sirs,
I have received your Parking Charge Notice (Ref: ______) for vehicle registration mark _______, in which you allege that the driver has incurred a parking charge. I note from your correspondence that you are not seeking to hold me liable as the registered keeper, under Schedule 4 of the Protection of Freedoms Act 2012. There is no obligation for me to name the driver and I will not be doing so. I am therefore unable to help you further with this matter, and look forward to your confirmation that the charge has been cancelled. If you choose to decline this appeal, you must issue a POPLA code.
Yours,
If appealing online, be careful not to tick and boxes/drop downs identifying the driver, and keep an eye on your spam email folder for a response.