First question, will equipment connected to the exported phase' be within reach of the locally connected equipment? (if not, there might be the possibility of creating two separate earthing systems in building B). OK, probably not, but worth a quick consider as designing out the problem can sometimes be simpler than solving it.

Use earth A up to the building but earth B inside the building.

Feels uncomfortable to me. What if the DNO temporarily disconnect building B's supply (complete with its earth) - the imported phase would still be live but without an earth at all.  Ditto for some maintenance within building B. Next best might be to completely TT building B - but that's probably a last resort.

Connect the two earth's together.

Probably talk to the DNO just in case, but in effect that's probably happening anyway via main bonding if the two building share extraneous-conductive-parts (e.g. metallic water or gas supplies). Bonding wise (and interconnecting protective conductor) would hsve to be sized for the worst case (e.g. the larger supply and for PME conditions).

    - Andy.

I did consider running the earth separately but decided it is not acceptable as there are some closely located db's which would potentially be on different earth's.  

I have seen elsewhere the argument that if one building is struck by lightning and the earths are connected surge currents could then be transmitted to the second site.  One of the buildings does have a real possibility of being struck by lightning (it's a church) and it has a lightning conductor.  However, I also take the point about the possibility that the earths may already be de facto connected via common services and will investigate this.  I will also investigate the connections to the lightning conductor.

I did consider running the earth separately but decided it is not acceptable as there are some closely located db's which would potentially be on different earth's.  

I have seen elsewhere the argument that if one building is struck by lightning and the earths are connected surge currents could then be transmitted to the second site.  One of the buildings does have a real possibility of being struck by lightning (it's a church) and it has a lightning conductor.  However, I also take the point about the possibility that the earths may already be de facto connected via common services and will investigate this.  I will also investigate the connections to the lightning conductor.