It simply doesn't have sufficient root room, that could be the main cause of the problem. I would suggest you pot on into something a fair bit bigger and particularly deeper with new potting soil, preferably John Innes No. 3 or a mix of that and multi purpose compost.

Thanks so much Bamboo. I can see now that it has such a long stem that it certainly needs a deeper pot. I'll take your advice and order some John Innes compost to mix with my multi-purpose.

It really needs to go in the ground this autumn, preferably...

Yes put it in the ground, they grow quickly, I had one set itself 2years ago but it was too close to another tree I planted. I moved it at the beginning of the year and it's doing really well.

Can I plant it on a slope?

If you repot it, don't use gravel in the bottom of the pot, though. That just stops the drainage. Just put plastic screen over the drainage holes.

You can plant it on a slope - it might grow at an angle if its a very steep slope, but no reason why a tree won't grow on a mild slope.

So many thanks for your tips.
Tug, won't plastic screening lead to poor drainage?

Counted cross stitch screen from the craft store is ideal, but plastic window screen is usually coarse enough.