Thank you all for your comments and suggestions. Snails are a sensitive subject in our house as they keep eating our courgettes in the garden.
I know they have nothing to do with the aquarium types but my other half gets cross at the mention of a snail.
So I went to my local MA this morning and got 6 Amano shrimps, as we were planning to get them for a long time anyway. They got to work on the algae straight away so it looks like I’ve got the tasty variety! I'll see how they get on in a few days. I hope my chain loaches don’t eat them and that they don’t escape. I'll post some pictures ‘after’, hopefully there will be something to show.
The cyanobacter grows at a corner where a light from the window catches the tank. The shop assistant from MA said that the combination of outside light and not doing the water change for two weeks contributed to this particular problem. I'll read up on it the algae on the page you've given, Sue. I'll also see if I can shade this corner more effectively, I can't really move the tank anywhere else.
The tank didn't have much green algae until I used Nitratex, so that might have caused some imbalance. A biggish water change is due after the holiday, so I'll do that. Nitrates are about 50 in my tap water, so I’m reluctant to do too big a change, as I will put them back in. I’ve got some biohome ultimate filter media in the filter, which has been there for a couple of months, so I hope the anaerobic bacteria will establish themselves there in a while and get on with processing nitrates. Not sure what else I could do?