AllyG, before using the skis that Ranchero has kindly given you, have the bindings tested on a binding test machine. This is the only way you can be sure that they are functioning properly; the shop should give you a printout with the values on it.
The type of fall that Trencher mentioned is called a phantom foot or slip catch fall, this type of fall seems to be the major cause of knee injuries, which account for about 40% of all skiing injuries. Female skiers are three times more likely to injure their ACL than male skiers.
IMO boot binding setup plays as significant role. A short skier in a small boot with a lot of forward lean combined with a binding that has a large delta angle (the angle formed by the difference in stand height between the toe and heel pieces) is likely to be forced into the 'back seat', making a backwards fall more likely.
A lot of research has been done into this type of injury, but little if any information trickles down to recreational skiers.
http://www.taosortho.com/Site/meniscus_pub_files/Skier%27s%20Knee-1.pdf
http://www.klokavskade.no/upload/Publication/Bere_2011_AJSM_Mechanisms%20of%20ACL%20injury%20in%20World%20Cup%20alpine%20skiing.pdf
jonG wrote: I got the injury skiing backwards whilst watching a friend learning and went down into a slight hole, the tails of the skis went into the snow up to the back of my bindings which threw me back into a sitting position quite hard and quickly and knee joint bent further than it's dessigned to do.....Imagine holding a piece of wood at the back of the knee then sitting down onto your heels hard, eventually youd pull the joint apart, thats kind of what I'd done....better posture and strengthened thigh and legs muscles would have probably prevented this injury to be honest.
Nasty, hope you make a full recovery.
