Update on this stuff.
2 sprayings of copper sulphate based on the dilution I found online. On hard or compacted surfaces it worked but not the case where there is grass. I spent a few hours the last couple of days on the kneeler scrapping it away from the lawn where there are gaps but it is wrapped around the grass roots so no way I'm going to eradicate it that way. I'm only doing this as the lawn needs mowing, but think if I do mow while there is so much algae remaining I risk spreading it, but will have to mow or will loose the hound shortly!
Decision is I will mow, then I'm going to ditch the sprayer & mix it in the watering can so make sure it really drenches around the roots of the grass where it is tangled. Hopefully this & good weather (not doing it this weekend obviously), will eradicate it. Even though the sun is shining there are some mighty slippery patches in my lawn so no way it can be left as it is or someone will take a tumble - that someone is likely to be me so more of a motive!