ps - how come your frame only takes one bottle, is it really small? My frame is quite small and has a heavily sloping top tube, but I can squeeze in two bottle holders but not full-size bottles. But I reckon 2 x 0.7 ml is still better than 1 x 1l. And I need both on a lot shorter (and easier ) rides than you're doing!

Another thing I thought of - I tend to have difficulty digesting yoghurt on the fly, but that's probably because I eat very little dairy products daily. Don't know if this might be the case for you or not.