Thanks DannyT. Right I see, I didn't realise that the rail I extracted would be so messy nor that its messyness would have an impact on my sweep operation.

Danny how did you break up the rail into 4? I don't know how to do this?

Is there anyway to cutdown on the messyness of extracted curves? I just took it for granted that the curve I extracted (via selecting the shells curve and copy and pasting) would be okay? I get the feeling that once an operation is performed on a surface it automatically creates masses of points? This is certainly something I wasn't expecting with Nurbs. For some reason I thought Nurbs would be immune from this type of thing...