Pare the rind off the lemons and toss them into a large (1 litre size) kilner jar. You really want unwaxed lemons for this, but don't worry if you can't get them, just give normal lemons a good scrub in hot soapy water and then rinse well and dry before paring off the rind.
Squeeze out the juice of 4 of the lemons and add this to the rinds in the jar.
Add the vidka.
Pour the water into a small pan and add the sugar. Warm slowly to dissolve the sugar and make a simple syrup. Let this cool.
Add the simple syrup to your jar and seal it.
Pop in the larder and give it a shake every day for 10 days.
Pour through a sieve lined with cheesecloth/coffee filter and bottle.