You need treat Cyanobacteria, which is what that is, with the antibiotic Erythromycin. You need a 5 day treatment and use the amount of Erythromycin daily for the size of your tank that is stated on the package. Many people use Maracyn to treat with but not sure what you have available in the UK. Erythromycin will not kill BB but you can get an ammonia spike from the dying Cyano
. Before starting treatment vacuum up as much of the Cyano
as you can. Also be sure to do the full 5 days of antibiotics.