Insurance companies (and the car dealers who use their services) are remarkably reluctant to advertise the real terms of their offers. (You'll nearly always find 'subject to status' in there somewhere). The only way you'll actually find out what the conditions are is to go to a dealer and ask them to make the appropriate phone calls.
I think, however, that you'll be very unlikely to get free insurance with a drink-driving conviction. The deal that the companies have with the insurance companies probably works something like this: The insurance company says to the car manufacturer "We reckon that the premiums for the people you'll be selling cars to will be between �500 and �1500 with an average of �1000. As long as you only take on customers within this range we'll actually only charge you �900 each. What we won't do, however, is take on anyone who's premium is above the upper limit unless the car manufacturer (or the dealer) pays the excess". So someone with a drink-driving conviction comes along and asks to be covered. The insurance company assess the driver's premium as �3000 (which, I understand is about the minimum for fully comprehensive cover on a new vehicle for a drink-driver). The insurance company don't actually refuse cover but they insist that the manufacturer (or the dealership) pay the additional �1500 (which will wipe out their profit). Result: No sale.
Sorry to be so negative!
Chris