Hi Budgie, my brother is in Perth, as well. Where were you treated? Being only 4 weeks post radiation, you will be just emerging from the worst of it. As others have said, don't expect the weight to come back any time soon - I think you've done very well just dropping 15kg!
My brother found carb soda, as suggested by Cheryl, helpful withe ulcers but also in keeping his oral environment better able to protect his teeth. Good luck with your recovery.