Vaillant water leak from bollier?

I have a Vaillent ecomax 824/2 and it works ok with just hot water but has problems when i turn on the heating. I first noticed a problem after the pressure gauge dropped and having topped it up twice it kept dropping after I put the heating on. I then noticed water coming out of the pressure relief value and also water dripping from the front of the bolier cover but have not taken the front off to see where it is coming from. I've read at lot on this site but am unsure of what may be the problem.