It needn't be elastic...just a belt, piece of rope, etc. Anything to reach around the ankle and to allow you to grasp the ends and pull back. You might try some heat on the area prior to exercising. Anything to gain the extra little bit. Good luck...you'll be at 90 degrees in no time. Are you doing prone knee bends? You should be able to incorporate them into your routine as well. Link below...
Thanks Eric, I managed to buy a Pilates strap yesterday, but unfortunately I just accepted the one the assistant took of the shelf. It was only when I got home that I noticed it was light strength......doh!!
On the plus side it was only £5 and my wife intends going to pilates classes soon so she can use it.
Next time I'll buy a stronger one.