Sounds like a mess.
This is not really Levies but instead a rather less formal fee for some sort of usage that is not in proportion to unit entitlements. As such, I don’t think that legislation that applies to actual proper levies can or must be applied.
Here is what I suggest. Since there seems to be no way to use the OC records to work out who has paid and who has not paid (of those who should) and the amounts are not trivial, perhaps the best approach is to appeal to people’s honesty, try to recover as much as you can and accept that a small number of people might dishonestly claim to have paid. The losses will hopefully be small.
Explain the situation in a letter and back it up with other communication so everyone knows that it can’t easily be overlooked.
Ask everyone to check their own financial records.
Ask those who have paid to confirm that in writing to the OC, ideally with evidence of payment. (Without spelling it out, it is clear that you can claim to have paid even if you can’t prove it. Some will honestly be sure they paid but their financial incompetence is such that they can’t find the evidence. You have no choice but to give them the benefit of the doubt. A few might boldly lie but they will have to actively write claiming that they paid while providing no evidence while knowing that everyone else will be providing evidence. A few might have the gall to do that but the unfairness is likely to be reduced. They will know who they are and they will know that you know they didn’t provide evidence. Most, I suggest, will not be so bold.)
Ask those who have not paid to please do so. Give them a new, longer time by which to pay to make it easy for them.
Near the end of the generous time, send a cover letter/summary to everyone saying that “As you may recall from our previous letter, we have been trying to resolve the … issue as fairly and equitably as possible. We are pleased to report that most units have now either provided evidence of earlier payment or have paid since that letter was sent. Could the remaining few units please remember check their records? These remaining units have details for payment and the relevant amounts on an invoice attached to this cover letter.”
I suggest that this approach will produce less inequity and less bad feeling if people feel that there has been openness, trust and honesty applied and most people have behaved well. It think it is likely that most will behave well.