blackthorn (sloe) berries in bangor / north wales?!
As Mactheboat said, look out for the white flowers, that are now in bloom (most places anyhow!), and then go back when they're in fruit.
Good places to look around Bangor are - the road going round the back of Tesco towards Caerhun and Glasinfryn, up Lon Popty and Hendrewen Road (again going towards Minffordd, Caerhun and Glasinfryn. Also try the back roads around Llandygai and Tal-y-Bont. There's probably plenty of trees around Tregarth as well. The little lanes are probably your best bet for Gwynedd and Anglesey.
Have fun with your gin :)
Oh yeah.........I've got a blackthorn in my back garden.........but you're not having that one ;)
Keep your eyes open now,for the distinctive white blossom of the blackthorn.It and wild plums,are usually the first to bloom. Take note of where you see the blossom and come back in Sept/Oct.
I would be very surprise that when you've discovered your sloes that when you go back a a local pensioner has picked your sloes clean as there are hardened sloe pickers pout the who lay in wait until their ready then strike on mass!