UKPassats

my 115 pd passat has started loosing water seem to be blowing it out of the over flow on the expansion bottle, i tried a new cap today to see if that would help ,when i set off from work the water level was in between the minimum and maximum and when i got back home (a 20 mile drive) it had blown more water out and when i open the cap the level has risen so it's now sitting above the maximum.
there are no other symptoms the car starts and runs fine with no overheating, no mayo in oil or water.
anything else i can check, how can i tell for sure it's a headgasket fail, also how bad of a diy job is it as i really can't afford this right now