From personal experience, you can tarmac over a concretre drive and the best thing is, because it has a base, it doesn't sink in the area where you park your car. Have seen drives where the tarmac has sunk underneath the weight of where a car is parked overnight, when it's been laid over topsoil.
You also need to take into account levels. You need to put a decent depth (a few of tarmac to make sure it lasts. Will this be higher than existing footpaths etc.
MPRSV1000 - do you have someone to do your drive? We are looking for someone to tarmac over our concreted drive, but can't seem to get anyone to do it?