After three (yes, three) attempts it dawned on me that I was using Bromothymol Blue by mistake (It is in a similar bottle to the Bromophenol blue) .
The fourth attempt, using Bromophenol Blue worked out fine.
So, best avoid Bromothymol blue altogether.
I'm dyslexic, I had to read that about 7 times to realise Bromothymol and Bromophenol are different things... I thought you'd made a mistake and repeated the same word each time :-o
