Adrian has obviously been creating his own tipple for a while now To explain some what he meant and also to reassure you, I can safely say that home brewed wines are nothing like toilet cleaner unless you muck them up, usually by putting too much sugar in.
To confirm the finish of fermentation you need a hydrometer. You can buy one online, search "homebrew". I personally like to use "thethriftyshopper", simply because the few times they do muck up an order they make it right pronto with no arguments. You could also get one in a homebrew shop or Wilkos.
Read your hydrometer according to the instructions enclosed and check again after 4 or 5 days. If the reading is the same it is finished. Pay attention to the temperture instructions. With experience you will learn to recognise it but, as adrian say, it MUST be below 1.000 or you could be making hand grenades instead of slurp.
Racking is just syhponing off the sediment. Do get a racking stick, available from Wilkos, online or a local homebrew store. This makes it so much easier and has a little cup to prevent the old decomposing yeast from being carried over.
Now this will probably cause a bit of controversy amongst other home brewers, but I feel keeping the demijohn topped up is not absolutely essential as long as you don't degas the wine. This is something you also need to know about. The reason that it is reccommended you keep the dj topped up is that it reduces contact with oxygen, which can turn your brew to something along the lines of bramble vinegar. A similar wine or, as adrian says, a little extra of the same wine fermented on the side can be used. I never use water. I'll take the risk with the CO2 in this case.
However, during the fermentation process, as you probably noticed, there is a lot of CO2 produced. This dissolves in the wine and lurks in the headspace, and being heavier than oxygen, displaces it. If you have extra to top up with then use it just in case (i know i do!).
Degassing. Very important if you don't want your wine tasting and smelling like a sewer, with a slight acidic sparkle across the tongue. Here you are removing the CO2 that has dissolved during fermentation. It also helps the wine to clear fully, since the gas bhoys up small particles. Take out your airlock, leave the bung in and cover the hole in the bung with your finger. Get a finger and thumb through the dj handle in case of slippage. Now shake the living daylights out of it. Carefully and slowly let your thumb off the bung or your wife will ban you from ever brewing again. Repeat until you get no more than a whisper of gas coming out. Do this after racking!
Good tutorial on brewing from the pulp, too late now, but useful for the future:
http://www.winesathome.co.uk/forum/showthread.php?t=1631Lots of good recipes here. Beware the american gallon though, you will need to convert, I've been caught out before by this!
http://winemaking.jackkeller.net/As for 3 bottles for a tenner...to hell with that! I can get 30 bottles for less than a tenner. Just have a look at the first tutorial in the wines at home tutorial series above. I do seriously reccommend you work through those tutorials if you are in any way interested in winemaking. They really help you walk before you run and avoid the dissapointment that can come of chucking sugar, frut and yeast in a bucket and hoping for the best.
Anyway, hope it turns out well for you. If not, post it to me and I'll drink it for ya ;)