Gaza, you know what, i'd never thought of that. I guess you could just temporarily detatch each membrane to see if that restored the tds to the lowest value, i guess that would tell you which membrane had failed, they last about 3-5 years so I'm told.
Stoner, adding another membrane will reduce your waste, but I believe that they reccomend only having a max of 3 membranes connected in series. If you havent got a pump I'd reccomend getting one as having a pump will also help to improve your RO's efficiency. The upgrade kit from Ro-Man is about 60 quid for a 100gpd membrane and accessories.
Tim