We have a few plum trees, and it could be that at only 4-5 years old, it just hasn't got round to flowering yet. Ours fruit in a two-year cycle. Those leaves look like a very bad case of fungal infection - ask Pimpernel - he spotted (lol) what the blackspot on my brussel sprouts was immediately, and this looks like a similar case, but obviously not brassica-specific. We live in a cherry-growing area, and lots of the trees round here display this sort of blight in Autumn each year, and are routinely sprayed with copper sulphate fungicide, but I'm sure there will be a more up-to-date remedy available. Sorry I can't be more knowledgeable or helpful.

I will have a good look later Karen I am off to work in a mo...Leaves are looking ropey on so many things just now. As long as it is not silverleaf or bacterial canker (and it isn't) the tree should be Ok. I am sure some one will put your mind at rest before I get back......

I can only make suggestions. Firstly, you have it sheltered, next to a south-west facing fence which is possibly the best situation. I would also feed it with just potassium in the summer. It could be late frosts that have burnt off the blossom in early spring and it could be that it needs a few years before fruiting. But the variety of plum might be crucial. Today, people are thinking more about local varieties of fruit trees when planting. I would give it another two years an if still no success, I would look for a variety which is known to thrive in your local area.

Regarding the non flowering, if it was a maiden tree on a reasonably dwarfing rootstock (in particular, Pixy rootstock) then you wouldn't expect it to flower for 4-5 years, so nothing unexpected there.
As for the infection on the leaves, looks like Plum Rust to me - rake up and dispose of fallen infected leaves as soon as possible, make sure none are left beneath the tree over winter, feed next Spring to maintain vigour.

Thanks everyone! I will pass this all on to my friend.

Hello Karen, I have gone over many images and Bamboo is right this is Plum Rust.

We have bonfire night coming up, and potash is a tonic for stone fruit. If you can get some from good clean untreated wood. It can work a little miracle.

Thanks Pimpernel! :)) Thank you for going over many images for me!

I am loving all the pests diseases on here...I can see what Pippa Greenwood likes about the topic. Give me decent pictures and the plant name...It is fascinating

Get yourself a copy of the RHS Pest and Diseases Book then, Pimpernel - fascinating stuff, that's where most of my info comes from. Hopefully there's an updated edition available which includes Leek Moth, Fuchsia Gall Mite and other more recent invaders.

Thanks Bamboo......Amazon on Sunday ! It is like another world within the Garden.