We gathered our sloes in the woods nearby where we live. Once you know what you're looking for you'll find that sloe bushes are all over the place in the woodlands and hedges here in the UK. Here is a handy online guide to recognising wild sloes, along with rosehips and blackberries.
My recipe for sloe gin comes from one of my absolute favourite and most used recipe books, The River Cottage Preserves Handbook. You will need 450g sloes, 450g granulated sugar and 600ml gin (it can be the cheap stuff!). Traditionally you are meant to prick all the sloes individually, but for reasons I don't completely understand, instead of doing this you can simply freeze them, which has the same effect. You then mix your sloes, gin and sugar in a large clean jar and put it somewhere cool and dark. Give it a shake every now and then to make sure the sugar dissolves.
I'll have to post some of my favourite sloe gin based drinks once we break open this lot later in the year. Happy preserving - and drinking!