I was determined not to let this pretty polysiphonous thing beat me so it was out with the razorblades. A quickie browse of the seaweed book suggested this might be P.fucoides, so when I keyed it out in a grown up key to P.nigrescens I was discombobulated briefly. When I realised they were the same thing all was well.
This is the second species this week that has had different names when keyed out! (Helina evecta
does not key out as Helina evecta, because it used to be called something else. Can't beat the stability of scientific names ...)
Anyway, Polysiphonia fucoides, ladies and gentlemen