First up, I found a stand of Damson (Prunus domestica ssp. institia var. damascena) near Little Preston (VC64). There were few fruit left and those were out of camera reach, so here is a picture from Wikimedia.
Next up was a dense suckering thicket of a small plum next to the allotments at Great Preston. While this plant keyed out to Black Bullace (Prunus domestica ssp. institia var. nigra) (see also below), I'm not entirely happy with the ID, the fruit were only just in the range for this taxon and the look of the plant was wrong with its densely suckering habitat (Bullace suckers, but not usually to this extent) producing a billowing stand more typical of Blackthorn (Prunus spinosa). I suspect it may be the hybrid between the two (Prunus x fruticans) but need to ponder further and perhaps revisit in the spring when it is in flower. The fruit were astringent but not to the mouth drying and puckering extent of Blackthorn.
Next up was a magnificent Hawthorn (Crataegus monogyna) on the way to Owl Wood, with exceptionally large haws. This is the perfectly named var. splendens. The following photo shows the size of the haws against three typical sized haws from an adjacent bush.
Brian Davis sent me photos of a couple of other plum varieties he found last year in hedges in VC31. These are included below to allow comparison. The first is Black Bullace, while the second is White Bullace (Prunus domestica ssp. institia var. syriaca).