Hi Maddy
Just found your thread. First of all there is nothing wrong with your hamstrings so please stop stretching them. Your hamstrings are spasming as a protective response to your back problem. You need to work out what you are doing when you extend your lumbar spine. If you are hinging from a particular level you will compromise the nerve root and this is probably the thing that is referring pain into your leg.
The fracture of the tibial plateau probably altered how you use your leg and this would compound your long standing back problem. You probably have a very good range of movement in your lumbar spine and possibly too much movement. Seek a physio who will analyse how you walk, weight bear through your old fracture side and change how you use your spine in a weight bearing position. Please note this is not 'core stability ' ( most spines are already too stable but thats another story!)as this will only serve to load the problematic spinal level .
Hope this helps
Steve