Thanks for the responses so far gents, I am not familiar with (approved doc B, or IIRC) being from north of the border, I always would advise that a risk assessment be carried out of any surrounding hazards that may be encountered during the evac walk to muster, in fact the large office complex I mentioned actually use a member of their security team to stop any traffic from entering their site(with the exception of course to the emergency services) during an evacuation so that persons can walk on the roadway as well as pavements to reach their muster points, and the road is not reopened until person are safely back in the building.
I have also encountered as you mention Chris, our local city council headquarters have a fair distance to go to reach their muster point(i would reckon about 200metres) crossing a busy road on the way,and another office complex I know of require their staff who evacuate using two particular exits to walk down one road and up another till they reach everyone else at the front of the building, which can take several minutes.