I done a bit of reading and research over this and there's allsorts of info out there. But i suppose a lot of it is "suck it and see".. I was tempted to try Citric Acid as a membrane descaler, seemed to tick all the boxes. Can't remember why i didn't try it, but it will be my next method to try. I did clean a membrane out with caustic soda. I disconnected the 40 40 membrane, tipped it on its side and filled it with caustic solution from a bucket. Half hour later i opened the tap and let all the fluid out..........it came out looking like a pint of Guiness . Definatley cleaned it, but the TDS went worse !! I.e it didn't do the membrane any good, so i bought a new membrane. When i let the fluid out, it spilt onto my leg, 18 months later i still have a big scar.
I would totally disagee with Spruce when he says along the lines of..." as the membrane gets older the pores get bigger and let more stuff through" (not verbatim) , i think that as they get older they get fouled and clogged with bacteria etc... so more goes to waste that way and less gets through.
So caustic soda is a big no no , or so i've experienced. Be good to see if anyone else would be willing to give a shot to trying summat else before they bin their membrane ?