Hi, David
Ordinary vodka should work, but I make "vodka" by buying 190 proof ethanol at a liquor store. Unsure what is available in the UK. I cut it half and half with tap water and keep it in a dropper bottle in a ziploc bag in my kit. A pipette would also work. I don't think dropping it from a rod would be as controllable. The exact dilution is not that important. I aim for about 100 proof (1/2 and 1/2 water and ethyl alcohol.)??
Alcohol won't insert sugars. It evaporates. And so does the water. The only point of the alcohol is to reduce the viscosity of the water so it soaks in more easily.
Ballistol is neat stuff for other purposes, but I prefer not to use anything which leaves residues in hammers or bushings.
Vodka makes the felt fluffier because when wool is dampened the fibers kink. Anyone who has tried to wash a wool sweater understands. So when the vodka has evaporated, which it does fairly quickly, the only thing left behind is the altered texture of the wool.
To take a sharp edge off the tone without gutting the power of the hammer or ruining its internal structure, put a few drops in the string grooves. If the grooves are very long and the hammers are very nasty-bright, use somewhat more. You'll find that a lump forms just past the ends of the string grooves. This is because where the grooves are, the flattened end of the previously oval hammer, the fibers of wool have broken and fallen off, but at the ends of the grooves they've just been packed in. The vodka gets them to swell again. So, if you've used vodka on that neglected a piano, those lumps can be removed with a few diagonal strokes of a hammer file, restoring an oval shape.
Gin smells like pine trees. The vodka just makes the piano smell like a bar room for awhile, till the fumes air out. Then no smell.
The biggest result from the smallest amount of vodka is right in the string grooves. You can also soak further down the shoulders, though if you need to go lower than about 11 a.m. to 1 p.m. I'd suggest gently squeezing the middle shoulders with vise grips instead. That's kind of drastic, for basket case hammers, but you can start gently.
If you soak the whole hammer, you've ruined it for anything except marshmallow playing, and the damage is permanent. It will get deformed. It's also possible to over-use steam. I prefer the vodka to steam (I call it "steam in a bottle") because it is more controllable, you don't need to haul along any gear except the small dropper bottle in the ziploc bag, and you won't get nasty steam burns.
Just one or two drops of vodka work especially well in octave 7, if you want a subtle reduction in "broken glass" or "tin can" while still leaving the twinkle-toes. Also, octave 7 can get a lot more even by using a very small amount of vodka only on offenders, leaving the rest. I like vodka in octave 7 because there is very little felt up there, and vodka removes none; also they are usually so lacquered up that needles have a hard time penetrating, and have less effect than further down. Poking little holes in something the consistency of wood doesn't really give a mellow result.