Hi Billabong, there are a few bits kind of sticking out along that boundary and there are a couple of tiny edges and one squished surface (looks like a line but is actually a surface with no surface area) that are making things complicated.

So to clean all that up I drew in a side profile and trimmed a thin strip away so everything would be cut to a common boundary, like this:



This way the area is filled by one trimmed surface like this:



- Michael