I have the same problem. The quarter inch turn may be a slight movement in the overall housing of the unit rather than the glass dome itself. Neff advised me to get a strap wrench like the ones used to open bottles or to heat up the oven to 50 degree C and try unscrewing (anti clockwise) with rubber gloves. I haven't tried the strap yet but heating the oven didn't work. I have even tried tapping the grips on the dome with a piece of wood and hammer. Still did not budge.
Martin Thomas, September 2009