What Mo has said is quite right, I've been waiting for 5 weeks for my licence to return so rang the DVLA last week, they say whilst the Medical Dept are dealing with the forms you are allowed to drive under your current licence as long as you haven't been told otherwise, they said (as my licence runs out next month) that if I don't get word back before my licence expires I would still be able to drive under Section 82 (?) so do not need to worry.
If your concerned give the DVLA a ring, in my own experience I find them fine to talk to and always helpful.