It keeps for some months in jars, I used to make loads of it and top up the jars with neat olive oil until I wanted it. I have made it when I lived in Kent, but it doesn't like my sandy slightly acid soil here. If you can find somewhere cool and dark to store it all it keeps for even longer, I have managed to keep it for a year in the past, but just occasionally it has fermented.
Yes, I am a forager too. I am so lucky here, I have a wood full of elderberries, so make elderberry cordial (wonderful for colds), elderflower cordial and sorbet, elderflower champagne and add them to hedgerow jam with the blackberries and crabapples. No mushrooms around though, sadly, although last year I found some morels growing in a bed I had barked, so I hope they will come up again.
I have never managed to find sloes in this area, either, apart from at roadsides, so have to make damson gin instead.