Changed 40-40 membrane 1 year ago.
Always run at 12 - 18 tds before DI.
Changed pre filters two weeks ago - lovely - TDS 7 before DI.
Just gone out there, TDS 089 before DI.
Baffled

Any ideas lads?
Cheers
Alan
Are you saying you left it nearly one year before changing pre-filters? If so, that is too long. It should be at least half yearly and possibly even every 3 - 4 months. If your sudden rise in TDS has been accompanied by a sharp drop in pure water production, you may have to get new membranes. That is the worse case scenario. However, it looks unlikely to be that because the problem has only occurred AFTER the pre-filter change. This causes me to wonder if the pre-filters are correctly seated within their housings with the rubber rings in place etc.
Another possibility is that there may have been a substantial drop in water pressure for some reason that may not be immediately obvious (the cause need not be immediately local to you).