Thanks Roger, Ken and Greg,
Yes, the telescope quite clearly has to be below the top of the walls. The alternative is not to include the front gable in the sliding roof, which negates the advantage of sliding the whole roof (also presents sealing problems). Since this will be more or less a remote setup (albeit in my backyard), I could probably reduce the pier height to 65 cm, and with the mounting plates and bolts etc, this will bring the telescope pivot point down to 150cm. This would clear a 2M wall height. Also having a flatter roof will be structurally stronger, lighter and more wind resistant. Wouldn't allow for a 150 scope down the track, however I'll be primarily doing narrowband anyway given it's in inner suburb in Perth. I have updated the plan:
http://130.95.21.121/pbrowne/Ob3.jpg
Thanks all for your advice!
Peter