Hi, I am currently struggling with surfacing on my leaf blower model outer casing where the handle blends into the body as one smooth part. I have used surface Lofts and Boundaries with tangent Faces to create this integrated handle and have managed to deal with the initial errors. 
 

My model is here : https://misse-my.sharepoint.com/:u:/g/personal/stephen_clark_miscs_com/ER6o7SD6g31AlKzb7cChG94BtT0NPegxNScykSZ5Fui0Lw?e=WQmSZ2 (open link in new page to download). 

I have now progressed so that I have appropriate surfaces defined, but it will not let me thicken the surface inwards more than 1mm.  Ideally I would like to make this 3mm thick, but the model is preventing this.  

I suspect that the area preventing thickening is the highlighted section below (as seen from the inside), however I do not understand what I need to do to resolve this problem area.
 

Here is the full side view:
 

Below is my original sketch of the blower.

Many thanks for taking the time to review this.  I will be grateful for any help and guidance to further my knowledge.