Thanks all for your comments so far. Hopefully as it grows it'll help with identifying it. Don't want to have a huge tree in the wrong place and upset the neighbours!
Schools with younger kids often have projects where they plant pips and stones from fruit they've eaten, to see if anything grows. It looks to be in the prunus family as already said, so I suspect it is a plum or cherry. Most fruit trees are grafted onto different rootstocks to control size, but as this is on its own roots, it has the potential to grow into a quite large tree (which will take many years) so you'll have plenty of time to chop it down if it gets too rampant. There's no telling what the fruit will taste like, being grown from a stone with unknown parentage, but you may be lucky and get something which tastes nice, eventually.
A trowel in the hand is worth a thousand lost under a bush.
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Hopefully as it grows it'll help with identifying it.
Don't want to have a huge tree in the wrong place and upset the neighbours!