Hi Michael,
I have attached a sample file.
A - Is the initial construction which is then translated to the solid B - no problem.
C - shows when the blend gap appears... you will see in the top view the only difference is that I have tapered the curve inwards, but by doing this you can no longer get the blends to meet up.
D - with this one i added some extra points to the curve and bingo it worked fine...
I'm guessing that blending is quite sensitive to the smoothness of the original line work that you draw.

All in all i've achieved what i wanted now and learnt quite a bit in the process, thank to all who have responded.

http://moi3d.com/forum/get_attachment.php?webtag=MOI&hash=6b0ece3447c8c0fbc9c0eb1d016d921b&filename=Test.3dm

Hi Cube,
I have looked at your file and see that you are blending in sections.
If you run the Rebuild command on you profiles and set the tolerance to 0.001
you will be able to blend as one surface.
If you look at the photo attached you will see I have added another model (right view top model)
where I have adjusted the top curve as it was to close to the extruded outer profile at
the one end and when blended was causing a small bulge.
Hope this helps.
Barry.

excellent! thank you very much. I had no idea you could do this. Ill take a look at your file later.

Edit - Just tried it and it works a treat, So why does a rebuild make it one continues edge rather than separate pieces (just out of interest)

Rebuild inserts more control points along the curve. Draw a curve then select show points, Now run rebuild on the curve by pressing TAB and typing Rebuild, watch the difference, it's instant!

Martin