Chris,

It looks as though 2 species are involved here - Wikipedia says the following about Zostera angustifolia: "This species is similar to Zostera marina.  Its leaves are narrower and are about 25 cm long and no more than 3 mm wide.  The sheaths form a tube around the stem.  The branched inflorescence is about 10 cm long.".

In the Online Atlas of the British & Irish Flora the following information is given:

"Zostera angustifolia (Narrow-leaved Eelgrass)

Z. angustifolia is a perennial which grows on sheltered tidal mudflats, in estuaries and in coastal lagoons, usually in shallower, more turbid water than Z. marina.   It is usually found on mud or muddy sands, between the half-tide and low-tide marks.

Z. angustifolia was not recognised as a distinct species by British botanists until described as Z. hornemanniana by Tutin (1936).   It was under-recorded in the 1962 Atlas, but its distribution is now better known.
World Distribution

. angustifolia is also recorded from Denmark and Sweden; it is difficult to distinguish from narrow-leaved variants of Z. marina and is not regarded as specifically distinct by Hartog (1970)."

It seems there may still be some doubt over the status of Z.angustifolia as a separate species - see here: 
http://www.ukmarinesac.org.uk/communiti … era/z9.htm

Steve