mo wrote:
I am about to repaint our concrete shed, along the bottom is has some
green growth - algae/moss or some other kind of crap

I had a Dulux masonry paint leaflet and they suggest using their product
to kill off this growth before painting

Any ideas on the cheapest way to do it - and is it only a temporary thing?

High pressure hose will do it. But it will come back if it's a damp area.
Maybe some of the silicone-based water-repellent treatments would help if
painted on the concrete. You'll have to have a look on the tin.

You could always try posting in uk.d-i-y - someone may already have the
answer there.

--
Jeff