Whiplash prognosis longer than assessed

In May this year, I had an RTA (not my fault) with the result that I received whiplash. I went through a 'no win/no fee' company and eventually was given a medical by an 'assessor' who assessed that my injuries would last about 5 months.

After the other chap's insurance company said that my injuries couldn't have lasted more than 'a few days' (some hopes!) they have eventually offered me £1750 (less all the fees I have to pay) - leaving me with £1,000.

However, I still have considerable pain 6 months down the line (in fact my summer was ruined and the effect of my holiday went out the window). I was given 5 lots of physiotherapy but not convinced that it helped much, also my car, which had been badly misaligned by the accident, was repaired.

I have to reply within 21 days to the other insurance company or they may retract their offer. The solicitor appointed by the 'no win/no fee' company has said that she advises that as I still have pain, I should have further assessment of my injuries, which might increase the offer.

I have other personal issues which are causing me worry so just wanted it over and done with, but now I am not sure. I was cross that they suggested that my injuries would only have lasted a 'few days' or that I was 'exaggerating' (I wasn't), and was willing to go to court to prove it.

My question is, if I have a further medical, (which is to see if the pain is due to anything else) and the prognosis date increases to say 8-12 months (I have no idea how long the pain will last), would the offer increase significantly or should I just accept what is offered? I have asked the solicitor about if there is a risk of my present offer being withdrawn if the medical report takes longer than the 21 days limit.

I have osteo and rheumatoid arthritis in various parts of my body (hip, fingers, knees, ankles) but never in my neck. Would the assessor say that the ongoing pain is because of arthritis developing? The pain from the accident has lessened since May but now seems to have settled into a certain constant level which never changes.