stuart noble writes
Can anyone suggest what may be attacking my pansies? They're in troughs
on a window sill and, one by one, something seems to be lopping the
tops off very cleanly at ground level. No soil or root disturbance, and
no damage to flowers or foliage.

Any advice welcome

Slugs.

They're active at night so you won't see any signs during the daytime,
but they eat through soft stems at ground level.

I don't know whether you're experience will be the same, but I've found
that violas don't get slug damage whereas pansies do. It seems quite
strange to me, since they're so closely related.

--
Kay