Sloes need a good combination of rain and sun during the spring and summer to get a good crop. It's not worth picking them until they're purplish and squashy. Many people say you shouldn't pick sloes until after the first frost - this helps to split their skin a bit so you don't need to bother pricking them to encourage the juice to seep out. You can get the same effect by freezing them in a bag for a short while.
Sloe gin is one of those drinks that it seems acceptable to drink at any time of day, a bit like a country version of sherry. A short nip is enjoyed by many before outdoor pursuits or to finish off a meal on a dark winter's day.
It's very easy to make. I have a lovely childhood memory of my father making it without a recipe, thinking it would be more complicated than it was. He boiled up the sloes with some sugar and gin, resulting in a sticky wedge of alcoholic purple jam. He did try to eat it on some toast but the sloe jam was too sour so he threw it away. Unfortunately our dog was a bin-raider and he decided to try it for himself too... not pretty.
To avoid this incident, all you need is a bottle of cheap gin and about half the weight of the sloes in sugar. Put the sloes (after freezing or pricking their skins) into a sterilised demijohn or bottles, add the gin and sugar. Seal the bottle and give a good shake, then store and shake regularly at first then every now and then. It's ready after a few months but matures and deepens in flavour over time. People argue whether you need to strain out the sloes and re-bottle then store, or leave them in for the maximum time.