Hi ian, I have the same system.... I found that one resin cannister (the first one) lasted 2 1/2months, the second I bought form ionics started to diminish after 3weeks!!! I wasn't best pleased at £60.00 a pop, so I went and bought my self a seperate DI vessel, 11L for £130.00 filled with resin after this I reckon resin will cost no more than £12-£15.00 per month at worst and you can buy the resin in bags to refill. I wont be buying any more DI cannisters from Ionics. Hope this helps.
In the meantime take your DI cannister off and give it a good shake up, this mixes the resin up and you will bring the tds down lower. On a spent cannister this needs doing every couple of days! hope this helps, Greg.