Ah - the additives
Yes, it can be confusing.
OK - crash course.
Citric acid (obtainable from just about anywhere) is just a more convenient version of lemon or orange juice. If you want to use those, the usual method is to squeeze a half or a whole lemon/orange as the equivalent to a half or full teaspoon of citric acid (that's a half or full LEVEL teaspoon).
Yeast nutrient is not absolutely necessary. If you're making wine from real fruit, it shouldn't be necessary at all. But if you're making wine from juices then there's not a lot in there to keep a yeast cell going. You can still do it, but the fermentation is likely to be slow. A level teaspoon of nutrient is worth using for that purpose.
Campden tablets are just a convenient form of sodium metabisulphite, which is an anti-bacterial and anti-oxidising agent. If you're careful and clean, it isn't necessary. If you're slapdash like me, it's probably a good idea.
Pectic enzyme is useful if you're making wine from high-pectin fruits such as apples (or even from jam). As you might expect, it destroys pectin cells and so prevents pectin hazes developing in your wine. Having said that, pectin hazes are not that common and, even if they do occur, are harmless.
Marmite is a brilliant source of Vitamin B1, which is a "yeast booster" which will speed up the entire fermentation process. I use B1 tablets for everything, but Marmite will do - but be careful with the amount as it does have a very distinctive taste.
Have I missed anything?
Mike
hi all
(although much slower now!) 
