Serevent and sinus problems

About 5-6 years ago I was taken off Ventolin and Becotide and put on Seretide inhaler. Generally it has worked very well and only use the inhaler now once in the morning and once at night BUT ever since changing I've had terrible sinus problems and can hardly breath through my nose at all sometimes. I can't put the symptoms down to anything else i.e. an allergy and wondered if anyone else has the same issue so I can confront my doctor with some facts! The stated side effects do include sinus irritation but my GP insists its a nasal polyp or some other problem so all I get prescribed is antibiotics and more steroids!