Hiya Jo, I don't think you can but I am not an expert!! Have you tried olbas oil and steam. There are some nose drops you can by "ovlatrine" or something like that, they will clear the stuffiest of noses!! Boots sell it from behind the counter!!
i find the water flavourings in hot water with soluble aspirin are great - just like a Lemsip or Beechams. If you really need anything else, you can ask the chemist for sugar-free versions. The normal cold remedies tend to be full of sugar but there are sugar-free versions for diabetics - usually hidden away though so you have to ask. Hope you get well soon.