Lewis,
I would use a 2 stage DI setup:
1 x 10 " 5 micron sediment filtre then
1 x 10" 250ml DI cannister.
OOOO........................just remembered

A Mate of mine, comes on here occasionaly - Phillip Thomas.
He stores his water out of his Merlin in rain butts at about 4ppm and he has got an old pair of tights, filled em with old resin and dropped it in the butt. Apparently it has brought the water down to 000

Yes he does DI in the van just to make sure, but worth a try

David