However, this Denniston's Superb has never lived up to its name. It took years to flower at all, and now only produces about 10% of the blossom I'd expect (making the Coe's Golden Drop look generous) and sets even less.
Festooning the branches has helped, and most of the fruit that sets is usually to be found on a bent branch. I usually summer prune, but still there is too much unproductive growth. This year I have removed all extension growth back to either a flowering spur, or 'knobbly' wood that looks like it might spur if it ever ceases to sulk. We shall see.
I suspect the real problem is that the soil here is unsuitable for plums. We have a very shallow layer of topsoil, on a few inches of clay which sits on water-logged gravel, the water table being only a couple of feet from the surface at times. I suspect there is a lot of nutrient leaching. Even the local blackthorn bushes set crops of sloes very rarely.