...but couldn't discover what it was. Not in books, nor on the web (I won't repeat some of the search words I resorted to). Just as well I've a couple of friends who seem to have an entire plant encyclopaedia in their heads! So...this is HORSETAIL (or the fertile stems of it anyway).
Honeycomb patterning on it's head (right) which becomes increasingly less defined as it ripens (left). Colours are light and breezy – sun bleached colours I thought? (wistfully). Full of healing and medicinal properties, Horsetail is also a really invasive weed I now discover, so hoping I didn't let any of its (murky green) spores loose in my garden or there'll be trouble...
(Thx P&B - who incidentally are also pretty good table tennis players).
...from my seasonal colour notebook.