Bought 4 packs of (different) seeds from this supplier.
The other 3 have germinated, producing many seedlings.
These rosemary seeds have not produced a single seedling. 0% germination.
Not going to be negative yet, perhaps rosemary seeds take longer to germinate.
It seems I planted the seeds far too late in the year for there to be any realistic chance of successful germination, but this notwithstanding one seed eventually germinated. I will try again closer to the time suggested on the packet.