Right, first things first, your mate needs to send the form back within the timeframe they allow (28 days?) to say that he has sold the vehicle, and give them your name and address as the new keeper.

Then they will write to you.

Do NOT NOT NOT NOT NOT admit you were driving, if you have the bollocks for it then go down the "someone test drove it" route and then proceed to prove to them that person has now left the country.

If you dont have the bottle for that, then when they right to you (which they cant do until after your mate sends the form back off) then just write back and say

"i have no idea who was driving my car 6 weeks ago im afraid, several of my friends drove the vehicle when i first got it as they were all interested to see what it was like to drive"

You will NOT get banned if you go down this route.