Help needed with irregularities in surface

Hi All,

I am pretty new to SolidWorks and I am trying to draw an oil sump pan. I used a lofted surface to draw the rear pan section of the sump and then thickened it.

The problem is that the surface has 2 "creases" in it and I can't figure out how to resolve this. I have tried altering the guidelines for the sketch a lot but perhaps I need to muck around with it even more. Alternatively, I may be using the wrong process altogether to draw this section, I'm really not sure.

I have attached a few photos for reference.

Thanks in advance for any help.

SolidworksSurfacing