I live in a very flat place (Norfolk) am I likely to be affected by cross winds, if I get a full frame bag?

Have you been affected by them?

I've not noticed anything untoward with a full frame bag.
I expect the extra weight your carrying in it keeps it planted.

Mornin' Tom

Indeed, N.falk may be flat... but the wind can make up for the lack of hills! That said, can't say I've ever noticed my framebag to make a difference.

I've used mine, along with a seat bag, in some fairly stiff cross winds and it only affects the bike slightly more, not much though. I've heard some people hypothesise it's because it's between the tyre contact points so it acts on them both equally or something like that anyway. You could always put some fabric of cardboard in the frame to replicate it so you know what's it like I guess.
Matt

i have noticed a bit of a difference; however saying that it is central so doesn't seem to twist the bike too much as you can notice with panniers.

also whatever you have; pannier, seat pack, frame bag or panniers will be effected.

Thanks all, Thomthumb I was drawing on my experience of panniers.

Well it doesn't help with my decision, full frame bag or tangle bag style bag?
:?

I certainly would let cross-winds affect your decision, the need to still use your bottle mounts is far more definitive.