A saline spray could help. But the one with Xylitol is probably better as although Xylitol is a sweetner made from a plant, it is also a natural antibiotic. The spray with Silver is also good. Believe it or not, Bromelain and MSM are also good for sinus pain. Don't know how they work, but they are natural.
Lemsip or any other painkiller like paracetamol, or panadol should be fine.
You could also try steam inhalations using Vick or Olbas oil. Some nasal sprays to relieve the congestion (the steam inhalations will help to loosen up congestion).
If you have a bacteial sinus infection, you will benefit from a course of penecillin or antibiotics.
Drink plenty of warm drinks. Keep off milk drinks as these cause mucus production.
I have been drinking Lemsip for the last week for a painful and infected sinus (mucus was green with blood). I am still blocked up, but as it started to spread down my respirtory tract (lost my voice and felt gurgly when breathing in and out) I decided to go to the out of hours doctor yesterday and now have amoxicillin (some people are allergic to amoxicillin).
Wishing you a speedy recovery.