In reply to airbournegrapefruit:
Good advice - if it is a large prolapse it can fill the spinal canal and put pressue on the spinal cord (stenosis, I believe is the technical term). Backwards flexion in that situation is definately not a good thing as it narrows the spinal canal even further and puts more pressure on the spinal cord.
Without an MRI, any specific or aggresive stretching is just guesswork and exposing yourself to risk.
Any loss of sensation 'between the legs' or control of bladder function, go straight to A&E (doubtless you have already been told this).
Keeping your core strong and mobile is really important though - I did lots of road biking (initially just short, really gentle sessions on the turbo - its hard to overstate the psychological benefit of just doing something initially), followed a proper fingerboard routine (strength based - day on, two days rest, really strict about intensity and quality), and amazingly, could kayak even when I could hardly walk (although the consultant did explain that if I were to come out of the boat and get bashed into any rocks it might not be a good thing - he was generally supportive though, and I decided the benefits outweighed the risk). I should add though that I had supportive friends who would carry my boat around for me so i didn't ever have to pick this up - that would definately not have been good news for the back.
Basically, as soon as you can, try to keep as active as possible and do things that don't stress your back too much or hurt.
Best wishes for a speedy recovery.