Exercise for ligament and muscle damage?

Geraint, please try this. There is a reflex in the body that allows a transfer of strength from the stronger muscle group to the weaker. So in your case this means that the uninjured shoulder has more strength. By working the uninjured shoulder you are going to increase the strength in the injured shoulder. So if you were to do lateral raises you would start by doing a set of 10 reps with the uninjured side and rest for a minute. Do 2 more sets this same way. Now do the injured side using only a weight that is comfortable for you. Don't try to push it as it will improve as it is capable over time. Do 3 sets the same as the uninjured side. Do every shoulder and arm exercise this way and it will increase the rate of recovery and strength.