Rhubarb Wine
6lb Rhubarb Rind of 1lemon
5lb Sugar 1 Gallon water
Half oz Isinglass 1oz yeast
1 slice toast
... is a fine example of what I'm always banging on about with old recipes

. People then made wine without really any idea what they were doing. It would normally taste awful, so they loaded it with sugar to disguise that taste. 5 lbs in a gallon has not a cat in Hell's chance of being fermented out, so that recipe would always end up sickly sweet. You've ended up using half that amount - much more realistic - but then added some more in the form of that sugar syrup you topped up with. So I suspect it'll still end up on the sweet side - just not so sickly as to turn your face inside out.
The bread thing is and has always been a fairy story. It may - only may - provide a bit of yeast food, but it's more likely to just give your wine a haze. So that's what the isinglass is for - it's a fining agent. Don't bother with it in future, as it usually isn't necessary. There are better fining agents these days and you don't need any of them until after the fermentation has finished.
Unless you're intending to make a sparkling wine (something I wouldn't try until you have a lot more experience), NEVER bottle any wine until you know it's completely finished and stable. That can take quite some time - certainly a LOT more than 5 days. So leave it in that demijohn until you can see it clearing. Keep it at room temperature to give the yeast a chance to ferment every bit of sugar it's capable of (and, in future, use wine yeast if you can. Bread yeast will produce a wine, but it won't produce quite as much alcohol as wine yeast).
Right - C.J.J. Berry. Most of his recipes are good. Some, probably because of supremely bad editing, are laughable. And he does come out with a few howlers. One of them is the full demijohn rubbish. For a start, demijohns should NEVER be full when you're starting a fermentation off. There is a distinct possibility of foaming and, if you have a full demijohn, there's only one place for the foam to go - all the way through your airlock and then all over the floor/table. For another thing, as soon as the fermentation begins, the yeast is producing carbon dioxide (that's what those bubbles are). The gas, being heavier than air, builds up a layer on top of the liquid, eventually filling the space left in the demijohn and providing a bug-and-bacteria-proof seal. When any foaming has died down, the demijohn can be topped up BUT even then it's never absolutely full. Leave a space of at least three quarters of an inch below the bung because, if you don't, you'll be putting one of the main sources of infection in contact with your wine. It's only if you're storing wine that the demijohn must be full (although still with that little gap) and then it's to prevent oxidation.
That frothy stuff on the top of your wine, by the way, has a 99.9% chance of being yeast - not everything you see developing around wine is bacteria.
Anyway, you've now done the right thing. However, top up your demijohn with tap water to replace that glass you drank (that'll help a little with the excess sweetness. Then just keep a casual eye on it. There's a lot of sugar and you've used bakers' yeast, so the fermentation may be slow and drawn out, but it will get there in the end. Believe me, you'll know when it's ready. The bubbling will have stopped and the wine will become clearer and, when you sample it again, there'll be no fizz on the tongue. Rack it again at that point (into another demijohn) top up as necessary, put the demijohn in the coolest place you can find and then leave it for another month. It's drinkable at this point, by the way - I'm just playing safe. After that month, it should be safe to bottle.
As a general rule, the safest way (and one which will improve your wine more quickly) is always to leave any wine for as long as possible in a demijohn, only bottling when you want to start drinking it.
Mike
EDIT: Elderflower WINE is easy. Elderflower CHAMPAGNE is not easy. And you're right about the cup of strong tea.