In reply to Mike Conlon:
Over the last 2 autumns, I've become fascinated by all things sloe and damson. In Wigan/Lancashire, I know of blackthorn bushes with no sloes at all, some with millions of small sloes, some with moderate amounts of large sloes (almost size of a small damson). Also, reckon Philip is right re: pollination. Luckily, I have sloes and tonnes of damsons, frozen and ready for use.
Out of interest, where are you based?
On a slightly different note, had an unusual blackberry season, which started with a few frizzled ones but ended with a frenzy of masses of big ones. Better than last year (and a lot later). I have a demijohn with 5 bottles of vodka topped up with the strained juice of blackberries and minimal sugar. So far, tastes amazing.