Only just seen this, it happens all over the place, the boating lake in Botanic Gardens in Southport used to suffer badly with it and it used to kill the fish. I worked in the Southport Branch of the Tool Hire company I work for back in the 90's and most summers Sefton Council used to hire a diaphragm pump from us. They pump around 400L a minute and are the kinds of things you use to empty a large pond, small lake, flooded cellar that kind of stuff.
Unlike the way its being used in the pic below, the council would set the pump up so that it drew the water out of the lake but had the outlet pointed back at the lake, so it then sprayed the water back in. They said this then oygenated the water in the lake
I've not been to the lake for around four weeks now so I don't know what state it's currently in, but it was still affected and closed last time I was there. I know the council have had to oxygenate the waters in some park lakes in Liverpool in the past, usually in prolonged spells of very warm, settled weather, which depletes dissolved oxygen levels.
I don't know if the blue-green algae was an issue when we were kids or whether we were just unaware of it, but these days it seems to be common in summer and, as you said, really widespread.
