"Wild" gardens are always more interesting - for me anyway
Pupae are normally quite difficult to find, since the larvae will tend to wander off and find a secluded site somewhere nearby to pupate. Having said that, I have found Small Tortoiseshell pupae on nettles before now.
When the numbers start to decrease, this is probably because the fully-grown larvae are pupating. Although there is some concern that Small Tortoiseshell succumb to parasites and disease, which may be the reason for the worrying decline in this species. But this could be a good time to place a few larvae in an airtight container with fresh nettle in (change each day) and with some absorbent material in the bottom. You'll then get to see the larvae prepare for pupation, where they'll hang upside-down in a "J" shape, and pupate within a couple of days.
Just remember to place the pupae somewhere where the adult can emerge safely and inflate its wings.
Cheers,
- Pete