Another thing to try is n-acetyl cysteine. It is mucolytic, so splits up the mucous so that it's thinner and so will drain. It's become available on prescription in the last 10 years or so - but the notification was on the back of the statutory instrument that told all the gp's not to work at weekends, so maybe your gp didn't read that far.
Holland+Barrett have it in big jars as a nutritional supplement, in capsules of 600mg. The sachets are 200mg each, and a French pharmacist told me to take 1-2, 2-3 times a day.
I used to have chronic sinusitis until a ventilation operation, and then it began again after dd was born. I discovered nac on holiday in France where they prefer it to antibiotics. Can be given to little ones, too - so we kept dd free from antibiotics for 3 years when she had glue ear.
Sadly, I now live on the edge of 2 weather systems, and when it changes from atlantic to continental and the air pressure alters I get what they call 'climatic sinusitis' - just as painful and horrid as the normal sort.
Good luck!
Could always go on a booze cruise and pick up a dozen boxes or so in cité europe 