wouldnt say it would be too big a problem but they can go any time, even when first done, bit of a gamble really. if the tank isnt going down it cant be losing any, it must just be condesation. whats the vdo? if there is water leaking near it how come the tank level is the same? or is the leak very small? the drops out the tank probably would be small airlocks clearing, did you bleed it when you changed the hose? not too sure how to do this on golfs as im more used to renaults. leaving the water filler cap off and have it ticking over give it a few revs may clear them through.