Hi both
thanks for the tips. I've decided to just state ~30 mins in the methods, not the most scientific but as I'm not actually attending the conference, another Phd student is taking it for me, so hence a slight panic that I'm not able to discuss my work. The actual data point isn't obvious that it was left longer than the others, so only I'd know it was different but I think I'm very thorough about everything is correct and I'm not hiding anything!
But I do like the footnote idea, and may use it in my thesis