There were actually two types that 'unscrew'. Look (or feel) up inside the car under the scuttle where the cap extends through the scuttle. If you see flats like a giant nut, you need to keep them from turning while you unscrew the cap.
On one type of cap, the 'nut' is part of the lower pipe, so just having the pipe still clamped to the tank may hold it strongly enough. But on the other type, it is just a nut, so has to be held directly. ISTR I used a large pair of channel-lock pliers.