Hi, somewhere by the boiler there will be a filling loop with a small quarter turn tap like a washing machine isolator tap, the filling loop is usually a braided steel hose, officially they are not supposed to be left connected full time, only connected for filling purposes, but more often than not they are left connected full time, mine always is!! As you fill the system keep your eye on the pressure gauge so as not to over fill!
Hope this helps!
STEVE, January 2010
