Solved
Merge Error
Morning All
Could somebody have a look at the attached model and help me to understand the merge error. I've tried several iterations of this and I cannot clear this.
Many Thanks.
I've been playing with similar models to understand this error and it stems from the open edges. If I extrude a small shape perpendicular through another cylinder, merge & trim there is no error. 
Offset the surface and the error appears so I think I understand what Creo is doing. 
I need to close my section, merge & trim then delete the surfaces I don't need! 