You may need a referral to an ENT to get sufficient treatment.
Many thanks Eric,
I'll have a chat to my GP here and see what he says.
If your mastoid cavity has granulation, you might consider using antibiotic/antifungal powder. There are various formulations. Simple includes boric acid (cheap and often effective), complex includes CSF-HC (chloramphenicol, sulfanilimide, amphotericin with hydrocortisone). Often these powders will help dry mastoid cavities. You may want to get another opinion. In severe cases revision mastoidectomy may need to be considered.
Hope this helps. Good luck!