Trod, the extra phosphate remover is a very good idea. My phosphate reading was practically 0 yet I had the same amount of cyano as you.
It appears we don't pick it up on the test kits because the cyano is using it. I doubled the normal dose of phosphate remover, and am still using that a month after the outbreak. I may swap to the normal dose next time I change it.
Just like you I was syphoning it out and within a few hrs it was back again. It really starts to get you down, but there will come a time when it goes. I was also blowing rocks with the Turkey baster (which I presume you will be doing) and I ended up hooving practically the top layer of sand off. I did drastically increase the numbers of Turbo / Astrea snails aswell. Some ceriths may be a good idea as they turn the sand as well as eating algae.
Stick in there, it will go soon.
Did you change the flow at all??