Did more reading about this plant and realize it's not practical to try and save seed, even though it had such pretty flower clusters forming. It's really difficult to germinate from seed so better to try carrying the plant over year to year.
As I don't want the plant putting all its energies into seed I won't use, I've cut it back some.
Saved the leaves to dry for a batch to put in my herb cupboard.
I had to rinse them to get some soil off and spread to dry a bit on a paper towel before bundling into a hanging brown paper bag.
The plant is now sitting on the kitchen windowsill in company of a thai basil.