Not sure if you have already solved this. To remove the yellow gear wheel, the gearbox needs to be locked. To do this, there are three options:
1. There is a guide on here on serviceing the gearbox on a Kenwood Chef Titanium. The guide explains using two wrenches - one to stall the gearbox at the vertical shaft afer removing the planet hub, while using the other to undo the yellow gear wheel. The risk of this method is that it is possible to ground out the vertical shaft with the wrench, meaning the planet hub might not fit back onto the vertical shaft.
2. Purchase a stall plug from Kenwood. This plug is inserted into the front attachment socket to stall the gearbox. You can then undo the yellow gear wheel with a wrench or spanner.
3. The other option is to use the butt-end of the handle of a large serving spoon. It must be a spoon with a strong handle. With the gearbox attached to the body of the machine, insert the handle into the front attachment socket, ensuring that the butt-end of the handle fits inside the slot, and that it catches on the pin inside the socket. While holding the spoon, use the wrench to turn the yellow gearwheel anti-clockwise to undo it. Refitting is the reverse of removal.
I hope this helps.