Okay. I couldn't see any diesel Volvos with a CO₂ figure of 99g/km on the HMRC site.
If your figures are right, then as you have worked out the annual tax amount then you simply divide it by 12 to find the monthly amount- or have I misunderstood the question?
I understood the tax system was changing for 2012/13 and that a higher rated D category would apply to diesel cars. You may want to check that.
Also, if you receive free fuel for your own use you a higher BIK tax charge applies. On the other hand if you have to pay anything out of net pay (eg a private mileage charge) you can offset that against the BIK figure