Cheers for the tips chaps.
What time do we get access to the hall in the morning?
It looks like it'll be best to do the straight 1750pts, 3 games then.
Although I'm going to add in a couple of rules to spice it up bit, well... perhaps 'rules that make people rethink their approach' is a better way of putting it.
1. All objectives have to be on/within buildings or ruins
(might need to rethink this if we don't have enough buildings/ruins though)2. Vehicles cannot drive over/through buildings or ruins unless they are skimmers.
3. Only models dismounted from vehicles can claim objectives.

That's the way that I'm thinking at the minute anyway.
If we are a bit short of buildings/ruins I could make rule 1: All Objectives must be at least 12" from the edge of the tabletop.
Although saying all that I might just write the scenarios so that these rules are incorporated into them so there is no need to specify how objectives work.
I'm thinking along the lines of 2 objective games and 1 Kill point game. I'm tempted to put a special objective in the kill point game though which would count as an extra KP to the player that destroys it - along the lines of destroy the 'uber widget' before it gets into enemy hands.