Fortunately, we had with us Prof. Rodgers of the Greenmount Institute of Horticultural Science (a.k.a John Shaw's friend and work colleague David, from Greenmount Agricultural College), who confirmed, after briefly checking his I-phone for, presumably, his research notes, that we were looking at species neanthes virens, or King Ragworm, which is cyclically distributed in these parts but which we had never come across before. King Ragworms can grow to around twenty inches and are apparently capable of delivering a nasty bite to humans from strong jaws which are pushed out on an eversible proboscis and normally used for pulling food into its mouth and defending itself. The 'feet' are actually gills - it remains in shallow water between tides and only feeds when fully submerged, managing to manoeuvre in the water by flexing its body into a series of S-shaped curves.
We left it to its own devices, and if you find one I suggest you do the same - unless of course Prof. Rodgers was pulling our legs.
***
Note: If, like me, you hadn't come across the word 'eversible', it means capable of being turned inside out - you learn all kinds of useful things on this blog.