Thank you both for the suggestions. I want to ensure that as the weather
warms, the main bacteria colonising the pond are not harmful. As you
say, one teaspoon of dormant/half dead floating bacteria are not going
to colonise a pond and process nitrogen in a hurry. I was hoping to hear
that people buy live cultured bacteria ready to go, but I guess it is
not used.

I changed the filter last year, so have no experience of it's over
winter retention of bacteria. The pond is four years old though, so
should have a good amount of nitrogen converting bacteria already, and
not need intervention.

I'm probably just paranoid. This time last year I lost quite a few fish.
The filter clogged, overflowed and half emptied the pond. As a result,
the filter was not turned-over for over 12 hours and the fish got very
stressed. In the next few days a bacterial infection took a few of them.
Thanks again,
David