Just did a virtual drive past on GSV and the signage is totally inadequate - in fact, there doesn't appear to be any signage on the side of the road where you were waiting.
The entry signage is also minimal.
You were clearly not parked and the operator's own evidence shows that.
This is clearly a very busy trading estate and it seems totally reasonable that drivers might have to wait a few minutes for parking spaces to become available at the individual units.
In the meantime, I would try and re-contact the manager and tell him that you have followed his advice but the parking operator has not yielded - he may be able to help.
As the driver is already identified then the only other option is go ahead with the POPLA Appeal - you have strong grounds.
Not withstanding all the above, they are unlikely to allow this to go to a Court hearing since the case demonstrates a very predatory nature.