yes, unless you live in a hard water area then you need to buy distilled water too!
(might still be cheaper, haven't done the math)
I've just topped up a bit, was sat at min mark, think it's probably just the well documented "sit on min line" issue, but I topped to max anyway so we'll see what happens!
I noticed some gunge however, around the exit pipe that goes from the TOP of the water box to smaller black pipe at the back of the engine cover, is redish in colour and was there before I did the top up, just around the edge of where the two pipes connect.
maybe I need some need clips to tighten the seal? or would you recommend a full system flush incase their is goo inside? (i'm thinking the only reason it has appeared to seep/gunge up there is because it might be stodgy inside the pipe causing more pressure to push the fluid out?
water resevoir is nice and clear tho, no gunge