I did a brief web search for "Poacea, corm"

and Melica spectabilis popped up,

no clue whether it could be a fit at all or whether it grows in Europe at all,

very remarkable, that corm thingy

Look at bulbous oat grass.

linaria - M. spectabilis appears to have 'bulbs' dotted along a rhizome, whereas these look like typical corms, like Crocosmia, Crocus, etc., albeit very small.

Embothrium - Interesting suggestion. Apparently Arrhenatherum elatius subsp. bulbosum is found in England.

I'll have to add this to the list of plants I need to keep an eye on, over the coming year.

Thanks to both of you.