Exercise Abdominal Cruncher (Brixton, London, SW2)

Im looking for an abdominal cruncher exercise machine. You lie on your back and reach to the bar and do abdominal crunches for your lower abs while your lower back remains on the floor. Its semi-circle with a padded head rest. Some have rests for your arms/elbows. I have a bad back and need to strengthen my abdominals. Many thanks, Linda