Well thanks to everyone for their response.....All your replies seem to
describe it well. I actually get rid of it by putting a paper kitchen towel
into my large net and swirling it around the surface - it kinda works!!!

I have actually removed the Ceratophylum and Riccia now - for a whole day
and it doesn't seem to have returned yet....However..we will see. I have
scrubbed the glass - though there was not much algae on that anyway - Ottos
Rule OK....

I will be taking further measures like a big water change this weekend and I
will be putting in more plants...

One outstanding problem is the Nitrate - it is 0 as is everything else -
what there is is taken immediately by the plants even though they are
planted in Flourite which should be supplying the nutrients.

Should I start supplementing with liqued Plant food ????

Thanks again

Martin