, Alisdair! Strange flower, indeed. I would not have got it if Joanna hadn't identified the genus.
Jill, apparently it is available as a plug plant from Thompson & Morgan. In their description they mention that it is "going down a storm in Dutch cut flower auctions and is set to be a favourite for modern wedding bouquets". They also say that " the pompon shaped green flowers are 5cm wide". So that must be the flower, though it doesn't look much like a dianthus flower. Who knows if it produces viable seed.