Agree with gingernut. The problem with knowingly flying with a blocked sinus cavity or eustachian tubes is that in the descent gunge can be forced in to the cavities if you manage to clear them.
You only need the decongestant if you experience pain when you didn't know they were blocked. In the case mentioned by Danny, the naturally small sinus entry holes can be ordinarily blocked by health mucus and reaming out will effect a permanent cure. This procedure is available on the NHS If your symptoms don't improve despite trying the treatments mentioned above, a type of surgery called functional endoscopic sinus surgery (FESS) may be recommended. This is a procedure to improve the drainage of mucus from your sinuses.