I have managed to work around the problem. I think the issue is a couple of things.
I wanted to sweep a profile, and made a guide line by creating a sketch and using intersection to create the guide line. However I do not think the profile accurately followed the guide line, as far as I could see. To get around this I extended the profiles edges.
This then meant that the swept profile intersected the face I was sweeping it along. I then created another sketch at 90 deg to the sweep to create an end cap for this profile. I then used intersection to get the profile of the sweep and the guide surface onto the sketch. But the resulting profile was not joined, and nothing I could do seemed to get a closed profile to create a patch. Even though the lines clearly formed a closed loop.
I managed to use a different method to create what I wanted, but on a lot of my drawings I'm still having trouble when either projecting sketches or intersecting profiles. I can create a line, use this to say extrude a surface, but then if I create a sketch and use intersect, the resulting sketch lines come out as several profiles, which then cannot be used for say a loft feature.
It seems that this is a fairly common issue and something that seriously needs looking at. My original query on this thread I think was caused due an error in the intersected lines were not recognised as being joined together.
