There are two things that can make wine "tart". Acids and tannins. I don't think your plum wine is overly tannic like a heavy red grape wine so that probably isn't. I would say that the problem is acids. Did you put your wine in a fridge for a few weeks to see if you can make the acid fall out...
I don't post much but have been brewing since about 1992. Actually, I got super involved in it and became a BJCP judge and brewed hundreds of batches of beer. I was never happy with the results in that I couldn't make a beer that tasted decent IMHO. I stopped in about 1998 actively brewing. Got...