This was living attached in Botryllus schlosseri on a very sheltered rocky shore. I've looked in Hayward & Ryland and on the web and it seems closest to Perophora listeri.
No interactions present.
A small ascidian doubtless, possibly even Perophora listeria which should be on stalks (Stolons or Stola) and be only 5mm tall.
http://en.wikipedia.org/wiki/Perophora (Wiki in the Panel fails)
Andy's own picture figures in Google Images
But we'll never know.
Lat/Lng: 50.2, -3.8
OS grid ref: SX7439