I rang Waterlife yesterday to see if it was advisable to continue to medicate with Myaxzin beyond 5 days; based on that my fish were still dying.
After a 10 min chat with a very helpful guy, he believed that there was something else going on.
We discussed feeding, water changes, new fish etc. He the described a behaviour that i assumed that was normal, ie fish "flicking off of the gravel". Based on this, he believed that i have "flukes", and that the cotton wool was a secondary infection.
So i'm going to medicate with sterazin once current treatment is over and filtered out the Myaxzin.
So fingers crossed.
A HUGE thanks to Knox213 and Wilder for your help (i'll put a good word in with Santa!!! lol)