Have a look at cherry plums and mirabelles on internet; they might fit the description.

Or maybe damsons?

We have a edge of wild plums (Bullaces) which look like your small purple ones. They are edible if a bit tart. They make a nice Jam which I like but my wife says it has too much tannin for her.
The yellow ones could be Mirabelles, but there are Yellow plums like Pershore Egg and others. Again very edible when ripe.

Thank you Jimmy, I have looked up Mirabelles and unfortunately they don't seem to match.......mine are not such a golden yellow and not quite so big, neither do they smell fragrant....in fact I'm not so keen to bite into the yellow/green ones. I have tried the purple plums and whilst they are perfectly edible they are a bit dry and not very appetising.
One of my neighbours has mentioned cherry plums so I think that's quite a possibility.
Thank you Sheilabub.....I think damsons are bitter but these are not?

Thank you Owdboggy, the purple and the yellow look very similar and so do the leaves.
I shall look into bullaces.....I could try the jam.....if the fruit is a little tart that might suit me better. I don't think they are the pershore egg plum as I think those are nearer to normal plum size?

Just as a foot note, Damsons are not bitter when properly ripe. They are not a sweet plum like Victoria or Marjorie, but perfectly edible.
The yellow ones could be a random seedling from a discarded plum stone.

And damsons make good jam . . . but best to remember to remove the stones first!

I seem to remember that a lot of the stones float to the top during boiling - but the memory is about 50years old so could be faulty...

I have made a very good jam from the small yellow ones.
Some of the stones were fished out & some left in. It wasn't too sweet, so go for it. They are often planted by councils in parks & around the neighbourhood( well in Surrey anyway), seems a shame to waste all the fruit they produce in a good year.

Oh, sorry I thought wrongly about the damsons then.......I certainly like the jam!
There is half a hedgeful of those yellow plum trees ~ or so it seems ~ the neighbour the other side used to have an orchard [where he lived previously] so he may know what they are. I think there could be 3 or 4 small trees of them in the hedge.
Absolutely Feverfew ~ it seems an awful waste just to leave them. I think the land here was an orchard to the church manse before this estate was built so the trees could have come from that?

Thank you all for your help ~ I shall see how many I can collect [was away for a couple of days so some have now gone mouldy on the ground] and then have a go at the jam ~ can I mix the purple and yellow for this? Wonder what colour jam it will make?

You'll have to let us know about the colour of the jam, Bluesky . . . I can't imagine it will be pretty!

Oh, dear..........do you think it will look murky?

No idea, but mixing purple and yellow paint makes a murky muddy colour . . .

Hmmm.....I better think again.....might even have to leave it till next year now as there are not enough of either to make something on their own.
Perhaps next year I could freeze them as they ripen as they don't seem to last long once they are picked.

It doesn't really matter what colour it is as long as it tastes nice though.

Yes.....true, steragram.....tasty jam is very good.