Neither fleet in beating to windward at the time of establishing an overlap (18.1a), Neither boat needs to tack at the mark (18.1b), The mark is not a continuing obstruction (18.1d).  

Some of the port running boats enter the zone at the end of their run, before the boats on the starboard fetch enter the zone, and therefore have established "mark-room" first.

Are they entitled to room to pass the mark and round up on port to commence the next leg of the course, or are the starboard tackers entitled to prevent them sailing their proper course to the next mark?

I concede that things are a bit fuzzy as to whether one boat is approaching the mark, and one is leaving (18.1c), since in the normal context they would both be leaving the mark to port or to starboard.

I say to port or to starboard, since the introduction to Rule 18 refers to leaving a mark on the same side, and in this case, although the mark is to port or to starboard, depending which way you were rounding, both fleets were passing on the same "northern" side of the mark albiet in different directions!

In reality everyone muddled through using pragmatism and there was no damage, but I would be intrigued to see what the forum's rules experts make of who is give way boat.

I carefully worded my question in that the boats were leaving the mark on the same side, the northern side, but in diferent directions ... surely the rule should say "leave the mark to the same side of the boat" or "in the same direction" if this is the intention of the rule?

Yep, I can accept this (excepting that there was no collision)