Passenger claim, going to court or not?

Hello,

my case is quite simple. November 2008 I was back seat passenger in a car that was hit by a vehicle coming from the opposing direction. 100% liability accepted by the other driver. As a result of the accident I hit my head in the front seat and I had a 15 mm cut just below my right eyebrow, a 4-5 cm bruise on my forehead (which then went down and cover all my face in blue-purplish-green for at least a month and now has left a whitish scare just below the hair line), a broken incisor and two damaged front teeth (just the surface). I could not remember if I was wearing a seat belt or not because of the head trauma, but most likely not, considering the injuries. I recovered the injuries, fixed teeth and I have two scares in my face (eyebrow and forehead) which are visible at chatting distance since I have short hair. I got a solicitor that worked with the insurance company of the driver of the car, since it seems really easy at the monent. I went to see a general practicioner and a dental expert selected by the solicitor company to complete a medical report (which took almost a year!). Now with all documents in hands my solitor has evaluated my damaged in this way: compensation 6500-8500£, finalcial loss 3700£. So a Part36 offer has been made to the other party insurance company of 12000£. The OS has rejected the offer and they are offering me 5300£ for compensation and 3700£ for financial loss, plus they will take off the 15% from the total because most likely I wasn't wearing a seatbealt. My solicitor says I should not accept because it's still too low and the prospective it's now to go to court. My question:  How do I know that the right evaluation of my damage has been made by my solitor and not by the OS? Is it worth going to court? what should i do? please suggestions!! :-) thanks a lot, Vale