Doesn't matter, Jacqui Too, whether the fosse complied at the time of installation. The normes change all the time, practically on a yearly basis, hence the requirement to have a fosse report on sale of a property.
If the fosse is relatively new and properly done, it is unlikely that it would have to be replaced completely. There might be small items like the ventilation pipe or the float (we had a float as we had a pump as well), or even the cover. Minor and not expensive to sort out.
Anyway, the lack of a report at this early stage of the sale should not slow down progress. You simply have a clause on the compromis to say "subject to fosse inspection". After all, the notaire's part takes about 3 months, so plenty of time to get the inspection and report done. Of course, it is then up to your buyers whether they want to proceed, renegotiate the price, whatever, once they have had sight of the report.
BTW, I wouldn't leave it all to the agent. If it were me, I'd make enquiries myself and see if I couldn't hurry things along. It might or might not work but my own experience has been that most officials are reasonable and can be persuaded to act if requested to do so on a personal visit.
Apprendre une langue, c'est faire un voyage différent chaque jour.
from Fle pour les curieux