Ratty, the film is expensive, but you may have to bite the bullet and spend some money on film to experiment with different lighting conditions if you want to use the camera/film properly and not get frustrated all the time.
On the Impossible site they do mention that the 2.0 film does seem to be faster. Also, even though 2.0 does not need to be shielded in most lighting conditions, I think it's still recommended
in full sun. I used to use a small black plastic bag (from B&W paper) taped to the camera to shield the film when it first came out (v 1.0) but then I got a frog tongue which made it easier. I still use it for the 2.0 version. You could do two photos in the same light - one with your normal procedure, one with immediate coverage, see if it makes a difference. If not, then you know that that's not the issue.
This
Flickr discussion has some good information about aperture and shutter speeds. The image below is from that same discussion (I'm adding it here as photos often disappear over there, making it useless for future reference). This is for folding SX-70 cameras: