I'm here, I'm the coriander queen! Wait until the seeds turn brown but by then they will already have been dropping off and you do realise you are going to have coriander self seeding everywhere in your greenhouse next year don't you?

I usually just put some sort of bowl or something underneath and rub off the dry brown seeds into it then pick through them later on to get rid of most of the stalks/bugs etc before storing the seed.
Hate to say it but my own home grown seed never has a lot of flavour, I presume it's the lack of heat during the growing season (tho' maybe not at the moment!), but as yours are in the greenhouse they must be baking hot. In any case you will have free seeds for sowing.