Thanks for your comments. The Vivatar is attached fairly close to the opening of the small softbox which is also quite close to the model. As I wanted the light focused on her upper body without too much spill I moved it in close which has probably given this hot spot.
I have since also bought a reflector which will help to fill in some areas. I'm still learning a lot on using strobes and all the possible modifiers. It's very exciting!!
Will very likely soon invest in a real "studio" strobe and some additional light modifiers ( beauty dish and a bigger softbox).
To clear some confusion, I made a mistake when subscribing to this site and used emmawest when my name is William:-)