Started on the replacement of the alternator rubber mounts this morning but have been unable to undo the two most difficult to access socket head bolts. 
Both sockets have been rounded so the 6mm allen key just turns in the socket. The easy access bolt came out with no problem. 
Has anyone got any ideas?

I have a set of 3/8" drive Torx bits.  When I ran into an Allen screw that the wrench didn't work in, I could loosen it by driving a Torx bit into the head of the screw and then using the ratchet.  It seems the geometry of the Torx bit grips into what remains of the Allen socket.  

Have you removed the battery to allow unimpeded access to the screws?

This is also a good time to mention that Allen keys are disposable and need to be replaced from time to time to prevent exactly the damage you are seeing.

#1 belting a torq bit into it - the hammering can help to loosen the corrosion.
Pre-hammering, i.e. laying into the thing before you even try to remove it, ideally with a flat-ended punch, can also tighten up a sloppy hexagon prior to using a normal bit.

Before attempting to loosen neglected fasteners, I always apply heat on them with a heat gun, spray Liquid Wrench around the heads, rap them on their heads then go have a café cubano and catch up on current events, bird watch or make snow angels. When I return, either the bolts crack loose or get another application of heat, penetrant, impact, repose and force until they do. In the last 30 years they always have; the 10 years before that were instructional. 

The basic lesson was don't try loosening a cold, neglected, unlubricated fastener by applying force with a dying strain. Using an electric impact wrench when possible helps break up imbedded corrosion but isn't a substitute for the heat-penentrant-repose part of the cycle preceding its use.

In this case, I'd apply heat and penetrant on the bolt, pound a suitably oversized Torx bit into the rounded fastener—as Point-Seven-five suggested and chris indicated—then go resume meditative activities. On return, I'd give it a try. If the fastener won't yield—or the over-sized bit slips instead—the fastener would be center-drilled with suitably-sized left-hand bit, heat, penetrant and impact applied, an easy-out inserted then extraction force applied.

Thank you all for the suggestions, I did remove the battery etc and used a new allen key socket and 3/8'' ratchet but I think a previous owner has attempted to remove the bolts and rounded out the two sockets. 
I've put it back together for now but will try hammering in a suitable torx socket as well as hitting them with a punch. Don't' think I'll be able to apply heat, they are so awkward to get at, particularly the lower one.
Thanks again for the advise, much appreciated.