Great Lakes
Always do right, not popular...
Most people use an empty plastic two liter soda bottle. You drill, or melt with a hot nail, a hole in the cap to accept an air line. The most common problem that can happen is that the CO2 gas escapes from this hole in the cap. A sure fire method of sealing the cap, (I can say, because it is what I have used) is to seal a short piece of rigid tube in the hole with the brand name glue Goop. The Consumers Union Magazine rated glues a few years back, and found that Goop was the only glue to reliably stick to polyethylene used in the bottle caps, etc.. Other people have also reported success using aquarium silicon and also with the use of a fitting sold for drip irrigation. In any case the seal must be air tight.
Also, it is highly recommended that you install a one way check valve in the line as cheap insurance against the potential risk of an accidental siphon. This is good practice for all air lines into the tank. I distribute it through the typical airstone, though there are many other practical ways to do this.
The recipe for the yeast mixture which I and others have used successfully is to fill the bottle half full of cold tap water. Add about two cups of white sugar and shake until most of it is dissolved. Then add 1/2 teaspoon of granular baking yeast. If you bake bread too, You can buy a 8 ounce bag at a wholesaler for about $5. It will last you forever, and I store it in a airtight plastic bin in the freezer. This yeast mixture does not activate for about a day, so mix it ahead of time, and hook it up to the tank the next day when it starts to produce gas. You can switch this mixture whether it needs it or not every other weekend, say during your water change routine. If you use too little sugar, it may not last two weeks. (I bet as little as 1 cup would do.)
Don't use too much yeast, as this leads to foaming, which will creep up the air line and go into the tank. The goal is to have a bubble every few seconds or so. I think that just allowing the bubble up in the tank is probably enough. Some people use fancier "beer yeast", which costs more, and the cheaper bread yeast works fine by all accounts. All in all, I think there is a lot of tolerance with this method and you should feel free to experiment.
Once you start with Co2, remember to keep an eye on your PH levels. If your hardness levels are low, PH can fluctuate.
Now, this whole set-up may cost you five dollars, so what are you waiting for
Also, it is highly recommended that you install a one way check valve in the line as cheap insurance against the potential risk of an accidental siphon. This is good practice for all air lines into the tank. I distribute it through the typical airstone, though there are many other practical ways to do this.
The recipe for the yeast mixture which I and others have used successfully is to fill the bottle half full of cold tap water. Add about two cups of white sugar and shake until most of it is dissolved. Then add 1/2 teaspoon of granular baking yeast. If you bake bread too, You can buy a 8 ounce bag at a wholesaler for about $5. It will last you forever, and I store it in a airtight plastic bin in the freezer. This yeast mixture does not activate for about a day, so mix it ahead of time, and hook it up to the tank the next day when it starts to produce gas. You can switch this mixture whether it needs it or not every other weekend, say during your water change routine. If you use too little sugar, it may not last two weeks. (I bet as little as 1 cup would do.)
Don't use too much yeast, as this leads to foaming, which will creep up the air line and go into the tank. The goal is to have a bubble every few seconds or so. I think that just allowing the bubble up in the tank is probably enough. Some people use fancier "beer yeast", which costs more, and the cheaper bread yeast works fine by all accounts. All in all, I think there is a lot of tolerance with this method and you should feel free to experiment.
Once you start with Co2, remember to keep an eye on your PH levels. If your hardness levels are low, PH can fluctuate.
Now, this whole set-up may cost you five dollars, so what are you waiting for
...unless u can find a UK site selling them 
