There is no sign for the motorcycle bay, only worn-out text on the carriageway. So if I park a motorcycle, do I have to pay or not ? All the carriageway text says is "Solo M/Cs". There is no word "only". It is marked on the carriageway as a separate bay, (see the double-dashed lines perpendicular to the kerb), so must have its own sign; they cannot escape from this requirement.
So submit reps on the basis that there are no signs to draw attention to the M/C bay, only worn-out text on the carriageway. The lack of a sign giving the conditions for this bay means the PCN cannot be enforced. Don't expect them to roll-over and cancel. Post-up your reps and their response when you get it. If you decide to take the matter to the Scottish adjudicators, you'll have to forego the discount option.
Also note that Scotland are still under the 1991 Act, not the later, Traffic Management Act of 2004.