Since I made this thread, I've found out a lot more about ab exercises. Try the bicycle crunch and do as many as you can for the first set, so for example if you did 20, try and do another 20, then another 20 then do as many as you can until failure on your last set.
20 reps, 5 second break, 20 reps, 5 second break, 20 reps, 5 second break then do as many as you can without failure.
Repeat that 'set' above 3 times.
If you can't do that, try the basic crunch below: