Fine grate rind of the 2 lemons into a large bowl or pot big enough to hold 5 litres of water. (Be careful not to add any of the white pith as this will make the beer bitter)
Squeeze in the juice of both the lemons.
Course grate the root ginger into the lemon rind and juice mixture.
Add all the sugar and cream of tartar to the mixture.
Add 1 litre of boiling water, stirring to dissolve the sugar. Allow to cool for 5 minutes.
Add 4 litres of room temperature water and stir it in. Using a clean finger test the temperature of the liquid - it should be warm, just about body temperature. (If the liquid is to hot or too cold the yeast will be killed as soon as it is poured in, ruining the batch and making it flat)
Sprinkle all the yeast over the top. Leave it for 5 mins to dissolve, then stir it in.
Add the 6 raisins to the liquid mixture and cover with clingfilm. Store your bowl/pot in a warm place, (but not in direct sunlight) for 4-5 hours. When the raisins have floated to the surface, the beer is done and ready to bottle.
Skim, sieve and stain the liquid into a separate bowl.
Decanter the 'clean' liquid into plastic bottles, making sure to leave about 2 inches of space at the top of each bottle, to allow for any excess gas to develop. We use plastic vs glass bottles because glass bottles can explore under the pressure as the liquid continues to carbonate.
Allow liquid to ferment in the bottles for 8 hours (overnight), but storing them in a warm place. (Not in direct sunlight, or in the fridge as chilling the liquid will stop the fermentation process)
Open each bottle very slowly in small increments to relieve any pressure from excess gas. Re-screw bottles tightly once pressure released. (Once opened the beer should be carbonated, if it is not, re-screw and allow to sit for a few more hours).
Chill in the fridge and enjoy for up to a week.