Please can you identify this fruit?

Could be a Malus fruit - which is a crabapple - this pic looks like Malus 'Golden Hornet' - was it growing on a small tree? And is the fruit soft to touch, or firm, even hard?

Looks like a yellow plum to me as you did say your neighbour let you have some of theirs is it the same colour and one might have fell and rotted and then seeded itself. plum leaves at edges are serrated

Looks like one of the little yellow mirabelle plums

It's possibly Prunus cerasifera or Myrobalan. There are yellow fruiting varieties such as 'Golden Sphere' which though originally grown in the Ukraine, are now becoming popular over here. They are very heavy cropping too.