Oh dear, you poor thing... cyanobacteria can be nasty. I wouldnt advocate using an antibiotic unless absolutely necessary. I guess the people who will know will be your local vet as to whether or not its available in the UK. Again they would be the people to ask if it would be invert friendly or not. There is a list of vets who will treat fish in the PFK magazine free shops guide in either last or this months issue. Not all vets have the knowledge to treat aquatic pets so i would advise getting some info from someone listed in the PFK guide.
The only invert safe antibacterial treatment i know of and use is Melafix - made of tea tree oil. This may or may not have an affect on the growth of it.
If it is definately cyanobacteria that you've got then i would advocate a drain down, clean out and start again approach...if you can bear it!
Good luck!