My RO has been creeping up the last month, from 12 to 22 and output dropping, so I was about to replace the membrane but decided to see if I could clean it instead and save myself some dollars. Turns out you can!
Removed membrane, bleached inside the housing, cleaned the outside of the membrane with bleach, then stuck the membrane in a long pot full of pure water and some HG washing machine descaler (whole bottle).
Left the membrane to soak overnight, reinserted back into housing, flushed for 30 mins and low and behold tds has dropped back to 12 and output is back to normal.
Not sure if this is a permanent fix or a short term remedy but if I get another 6 months or a year out of the membrane, then its definitely worth the £5 spend on the descaler for anyone who wants to try it!