I hope someone can help..

I'm having a constant issue with several guitars at the mo..

I'm playing in drop c so have put on a different gauge string (9-42's) and a different tuning from how the guitars were when I first got them and in order to avoid getting fret buzz on the first few frets it means I'm raising the bridge so much that the strings are so far away from the body the higher up the neck it goes..

It's the same situation on Floyd Rose bridges, les paul tail pieces, all in one bridges, strat bridges.. the lot! I've checked out lots of videos online about set ups and the intonation at fret 12 is the same as the open string so it's good in that recard, and as for altering the truss rod.. it seems like the neck is straight..

Is there a quick fix or something silly I'm doing wrong?

Thanks A LOT!!!!
Something silly.

9-42 strings are too thin for a tuning such as drop C. You aren't getting enough tension. Before you go about trying to set up your guitar for this tuning, consider trying thicker strings. Skinny top and heavy bottom strings would work much better.
"getting fret buzz on the first few frets"

you need neck relief. you neck should not be dead flat and in fact you probably have a back bow. first do what the wizzard says and get some heavier strings on there. then if the increased tension hasn't eliminated most of the buzzing, you'll need to dial in some fretboard relief.