Ok guys thanks, I'll get some Milton tomorrow.
Can only think the reason for the algae is white plastic tank. Grows in summer then killed off in winter hence my water being full of it. It's almost microscopic mind, I'm not talking chunks of the stuff but my strainer is very, very fine mesh.
It's happened to me before but emty tank & a good flush with hose has sorted it in the past, not this time though.
![Sad :(](http://www.cleanitup.co.uk/smf/Smileys/classic/sad.gif)
My tank is filled from mains as I'm DI only, on demand.