Malcolm, I laughed at your Quatermass comparison, must be quite the stuff you have there. It may just take over the whole of dear old Blighty!
I did a bit more research and though I did not come up with anything definite for you, below is a list of types of algae, all have hundreds of species, but these classes of algae are found in freshwater.

Try Googling them if you like and see what you come up with. That should keep you out of mischief for quite a time. lol

Here you go.

Cyanophyta.[Blue-green] Freshwater examples= Nostoc, Anabaena.

Chrysophyta. [Yellow-green] Freshwater examples = Vaucheria, Tribonema, diatoms.

Chlorophyta. [Green algae] Freshwater examples = Spirogyra, Volvox, desmids.

There are also reds and browns but they are mostly marine types.

Anyway give them a spin and see what is there.
You maybe just referred back to Professor Quatermass;-)
Take care,
"Horton"

P.S. Hi Sue [Scotty] hope you are well!
"Horton"