I know that apple trees need another one in the vicinity to fertilize the flowers but I'm not sure about other fruit trees.

The last two years have been hard for fruit trees in many areas. Last year was dry in spring and then wet all summer, so any fruit which formed may well have dropped early. This year was a long cold start, preventing pollinating insects from getting about their work. As said above, pollination requires trees of a compatible sort to be around, i.e. flowering at the same time as yours, obviously plums for plums and apples for apples. Even self fertile varieties perform better when there are other suitable trees nearby. It'll happen.

thanks all .last year there was a few fruits as i covered the tree with a net at blossom time ,i thought the bullfinches would eat the blooms but the fruits dried up 7 fell off ,this year although lovely blossom nothing at all ??

If its any consolation Pete my plum has nothing at all this year and the apples were overloaded but with this prolonged heatwave and no rain the June wilt is still going on and at this rate there won't be any apples left on my trees come Autumn, its the way things happen I'm afraid...

Worthy, damsons don't need a pollinator do they? Please say no... I get lots of blossom but hardly any fruit sets.

I've got a few apples in the second year of planting but it's a nursery-bought plant, supposedly self-fertile variety 'Scrumptious' and there's a crab apple not too far away so I guess some cross pollination may have happened anyway. During the first year everything dropped, though I heard that's not unusual with young trees on dwarfing rootstocks while they are getting established.