I don't think it matters if you injured yourself at work or not. Could your employer claim your injury is the result of not following health and safety rules ? e.g. were you given proper training in the handling of heavy goods ?
If you were, they could claim you ignored the training and could have avoided the injury by being more careful, thus avoiding all responsibility.

As for what you can and can't claim, there will be government guidelines (which are not usually in your favour), otherwise, it should be spelt out in your employment contract and terms of engagement.
Time to read the small print methinks