Gardiners have the merlin on special at £220 +vat + free delivery at the moment
the only difference in the pics is the sizes, I've the 11 ltr one like in the first pic and I run that with a merlin, I produce 250 ltr a day on mains water of 350-400 tds, product is 030 ppm before di, tds has only crept up to 002 now after 6 month daily use. The bigger the di vessel the less frequently you will have to change the resin. I bought a re conditioned vessel empty from cleantech, only about £50 I think.
Simon