I can get a 142g footprint for my tent that allows it to pitch fly only shedding a reasonable bit of weight. Obviously it's not gonna be midge proof and there is a bit more of a chance of weather getting blown under but am I right in assuming it should keep me dry enough to not have to resort to a bivi bag (thereby not saving any weight)?

I've never tried a tent in this way before so would appreciate any advice if anyone has.

It should work fine, just make sure the footprint is a good few inches inside the fly.

It'd have to be really nasty for much to get blown underneath. I sometimes use a tarp with no bivvy bag and haven't ever really suffered with stuff getting wet.

EDIT - I should just say that you may suffer with condensation a bit more than usual, so if conditions allow try and leave the door open a bit. Any condensation was always present but the inner protected you from it

sounds perfect ta, I'll order one

Might be of use:

http://sixmoondesigns.com/faq/shelter-faq.html

and

http://sixmoondesigns.com/faq/tips-a-tecniques.html

Interesting, they do some nice looking tents too but I managed to resist