right i have checked and its 515 before the membranes, and 031 after and before the di filter. does this confirm your theory?
Possibly but not necessarily. Sorry I can't be more helpful than that. If the TDS is climbing a lot more sharply than usual and this has been a fairly sudden change then it may well be the membranes. If this is the problem, it may be accompanied by a fairly sudden drop in pure water production rate as well.
Usually you can reckon on changing the pre-filters every six months (possibly less) and the membranes every two years. This can vary but that's the general idea. Do you know when you last changed the pre-filters? Presumably you have never changed the membranes in view of your original post?
It seems odd that your TDS reading after DI is 017 when you have only recently changed the resin and when the TDS after RO is 031. At 031 TDS you should be able to get 15,000 or more litres from a 25 litre bag. Would your DI unit happen be a very small one? For instance, if it holds just 2 litres of resin, at 031 TDS, you would maybe get 1,200 - 1,400 litres of pure water before needing to change it.