Hi Mog,
I hope you are recovering well from your injuries and things are getting a little better for you. As Martyn has said above technically your employers can not sack you or make you redundant for making a claim for personal injury. However if you are unfit to carry on with your job then they may be able to make you redundant due to this.
However I think your concern should really be how the injury is affecting you, if you are unable to return to work and indeed as a result no doubt losing earnings then making a claim for personal injury may actually help you more than hinder you. Making a claim for personal injury is not simply for monetary compensation. As a part of the claim you are also able to obtain private medical care, therefore if indeed the injury has made your knee replacement more painful and further surgery is required in the future you will get this done without the need for waiting on the NHS waiting lists.
In addition to the above if this accident has renderred you unable to work you will be able to recover future loss of earnings also, however this may be limited to the statutory age of retirement however if you have a strong claim that you would have worked longer then this will be considered.
So in making a claim the worry about the job being secure really is not something of significance is you can not undertake the task in any event. I would recommend you find a solicitor to deal with your claim for you and we would be happy to discuss the matter with you further if you wish, either click on our add to the right of this post or the link below and see how we can help.