When to harvest pampas grass seed

Don't think it readily sets seed in this country (probably just as well!); plants are also either male or female and you obviously need both.

Although both do produce plumes the female ones are much showier than the male ones, so even if you were to get viable seeds you wouldn't know whether your new plants were male or female until they flowered (probably a few years!)

For that reason pampas grass is normally propagated by dividing new plants off from the (female) parent plant.

Well I bought seed this year and although a bit difficult to propogaste, they have grown well.  The seed I bought was from the UK, so some people are collecting it.  Just wanted to know when to do this

I don't really understand your question; you seem to be saying you have mature pampas plants grown this year from seed (i.e. they are flowering) and want to collect more seeds from them?

If you already have viable seeds and want more plants why not just sow some more of them?

Surely you would wait until the seed head has matured, as you would do if you wanted to harvest any grass seed?