Yes, Jenny is a self-fertile variety and I would say that your hand-pollinating should help. Do you feed it too much? If this the case, I would only feed a potash fertiliser as it might be putting on leafy growth at expense of fruit. If no one can come up with an answer, I would suggest buying another plant, 'Solissimo' which is an improvement on Jenny if you are still keen.

Possibly it was a bit dry at the root when fruit initiation should have taken place? - if that happens, fertilised flowers will abort.

Thank you both - I replied to this the other day, but it doesn't seem to have been listed!

I had given it a spadeful of garden compost as well as a good scattering of potash. So it may of been too much I guess. Also it could of been dry - it is in a slightly raised border with a path below. We did have quite dry, cold weather last spring. But we are situated in the bottom of a river valley, so it shouldn't really dry out.
I will try giving it just the potash this spring and make sure it doesn't dry out (no chance of that this year!) and I will pick up a Solissimo when I see one.
It had lots of flowers - so I will try hand pollinating again too.
All the other fruit did really well, raspberries, blueberries, goosegogs etc... Even had a first crop of quince.

Its quite amazing how quickly soil dries out, even if its waterlogged in early spring. As its nearly always windy now, that dries things out quite a lot, and once there's a bit of sun on the area too, within a couple of weeks, it won't be wet, and possibly not even damp, any more...

Hoya, you just reminded me..I had a 'Jenny' once. It grew like stink but never any fruit, so after about five years I decided to cut it down. Blow me down if there weren't fruit right at the top when I pulled it down off my wall! So check up high!

Oh, I will! It reminds me of my Mulberry tree, looked at it for years thinking 'No fruit yet'. Then when I was weeding underneath I looked up and the whole tree was full of fruit - they just don't show at all from the outside!

:) mulberry tree...i've only ever seen one in Chelsea Physic garden...beautiful!

It is gorgeous - as is the mulberry vodka that I make from it!

Oh, now you're talking!...but I do prefer Gin tbh. ;)