Your medico legal report should have given a prognosis - ie "full resolution in x months if a course of y physio sessions are completed". The offer will be based on that. Your difficulty is that everyone responds differently to treatment so the injury could be completely resolved within the time estimate in the report or it could take longer. If it resolves within that time fine, but if it takes far longer you will not have been adequately compensated for your pain suffering and loss of amenity. The award for psla will be based partly on the extent of injury and partly on the length of time it takes you to recover. Your injury will be in the lowest bracket of the Judicial Studies Board guidelines which indicates an injury such as this resolving within 2 years should attract an award of between 750 and 2800 (I think).
I take it the implications of a p36 offer has been explained to you? If you accept it, you are entitled to your costs up to acceptance. If you reject it and fail to beat the offer at trial, the other party are entitled to their costs from the last day on which you could have accepted it. Although in your case, I suspect costs will not be an issue for you, since normally the insurance co picks up the tab or you will have ATE insurance. You should check the position though.
If you wished you could put in a counter offer. Its a difficult call for you though, because only you know how much you have improved since July.