I just to a postion of cure disease and it didn't say anything along the lines of Helljoint cured etc. Does this mean the potion did nothing?
If you've swallowed a potion, you'll be cured. The game is very vague about this, tho...it won't tell you you've been cured.
IN the future, just look at your stats. If you've contracted a disease, one or more of your stats will be lower than usual, and will also be red.

In this case, I think your agility will be lowered due to helljoint. After you swallow the potion, you can look at your stats again, and they should be back where they were before the disease.