does a named driver have the same cover as the policy holder ie. fully comprehensive

Yes, a named driver will have the same level of cover as the policy holder (main driver). So if you have fully comp policy for the car, all the drivers listed on the policy will benefit from comprehensive cover.

There are 2 things which are quite distinctive when it comes to named driver. First they are supposed to use the insured car every now and then and not most of the time. It is only the main driver who has use of the car most often. Second is that named driver usually do not earn no claims bonus (NCB), this only goes to the policy holder.