Please ask your questions in the comments section below. I will answer them there and if they haven't been asked before I might add them to the body of this page.
Bananas are a good source of potassium and yes, they do add body to a wine though I don't think the potassium has much to do with that. They are also a natural source of the enzyme amylase which breaks down starches into fermentable sugars. This helps the wine to fall clear. The only downside of adding bananas can be excessive frothing when the fermentation is at the vigorous stage.
Your article said that it's the grape skins that add a dark colour to the final wine. I was wondering if I could add some crushed dark grapes to the juice in the demijohn to gain a darker colour? Will the natural yeasts be a problem if using sachet yeast?
You could do this but as well as colour (and flavour) you'll also extract tannins from the skins. Tannins can make a young wine too harsh to drink. The wine won't be spoilt, in fact it could be better than simple juice wine, but you'll want to mature it for longer before drinking it. The natural yeasts probably would cause no problem but to be safe, I would flash rinse the skins in boiling water before adding them to the must.
As I might make a couple of batches, are their any other tips on flavours (I like a dry, rich red wine not a sweet wine)? A clove or cinnamon stick might ruin it I'd say. I've heard of a teaspoon of honey added to feed the yeast?
Cloves and cinnamon are for adding to warm mulled wine at Christmas time but not for putting in the fermentation jar! Honey is not a yeast nutrient but does no harm. The world's best wine is made from - grapes, and only grapes. The best advice is, use quality fruit and juice, pay attention to cleanliness and temperature, don't be in a hurry, and read all the articles on this web site!