Hi Kev
The carbon filter on the merlin should be changed every 19,000 litres of water used. In some cases this could be as often as very couple of weeks if you use a lot of water.
The carbon filter is important as it removes the chlorine from the water and it is the chlorine which damages the membranes.
I would change your carbon filter and if you have any extra sediment filters change them also and see how that affects your tds levels, hopefully your membranes are still ok. If that doesn't work give us a shout and we will try to help.
Arthur