Hi,
I don’t know if it is an easy fix but if your mixer is a Chef model A701 or A 701a here’s the [http://www.britishexplorers.com/Kenwood%20gearbox/A701a%20service%20info.pdf|service manual] (be patient as it is a slow download)
On p.3 it describes how to dismantle the Planet Hub and beater fixing assembly. The numbers in brackets in the description are the parts of the assembly as shown on p.16.
Also on p.3 it has a list of the tools necessary for repairing the mixer which may also help.
Hopefully this will help you to re-assemble it again.