There is no catch all spray solution to the problems you're having - its necessary to diagnose the problem with each individual fruit tree and then, if possible, treat for whatever the problem is.

If your Pear tree produces scabby fruit, it may have Pear Scab, but you'd see black scabby marks on the leaves as well. There is unfortunately no treatment available for this fungal infection,more information here about that https://www.rhs.org.uk/Advice/profile?PID=81.

With regard to the plum tree blossoming but not fruiting, this could be because it flowers early and the flowers are damaged by frost before pollination or fertilisation, or there aren't enough pollinating insects about when its in flower, or it may have a problem with Blossom Wilt, another fungal infection, details of that here https://www.rhs.org.uk/Advice/profile?pid=92. If it is blossom wilt, you'd know because the blossoms wither and shrivel but remain attached to the tree.

Thank you very much Bamboo.

In February you may spray the trees with a Winter Wash (organic as it is a horticultural soft soap) to kill off the eggs of overwintering bugs when there are no bees around. Helps reduce Codlin moth and Plum moth too.