Is it? should it be there? it looks exactly like the mosquito larvae I
used
to buy frozen in blocks to feed to my tropical fish. Maybe it's all the
hot
weather we are having here in the UK.

This same question apears every year. Usually they are Black fly larvae.
Take a look at
http://www.sacs.k12.in.us/woodside/t...s/blackfly.htm or
http://creatures.ifas.ufl.edu/livestock/bfly.htm#intro.

I'm in Essex and get them every year, just scrape the little beggers off and
let the fish eat them.
David