I had a similar problem like this last year and came on here for advice and was told to re pot with better quality soil. So that is exactly what I did. But now bonsai is grumpy again. Any advice please? The soil is damp and it gets sunlight through the window.

Did you move it from another location recently?

Give it a couple of weeks and it will probably make lots of new shoots.

It could be changes in weather, humidity, location. how did the tree look after the repot? like your Profile pic?

you could carfeully look to see if it is root bound. if so, I would possibly slip-pot the tree in a larger container.

It hasn't been moved but it is winter here in the UK so temperatures have dropped and less sun. But what else can you do, take him on holiday?!

I think my profile picture is before I lost its leaves last time. But since being re potted it has been perfect. Until now.

Is this a normal yearly thing and not something I should worry about?

This is not normal and you should be worried about it.

It could be due to various reasons, all related the plant not being happy. Merely reduced light is not the biggie.

I am going for, in order of reduced likeliness for me:

Rootball problems due to drying out (Does it sit on a heater?) and subsequent wetness
Extremely dry air indoors
Bug infection (Scale, spider mite the most likely culprits).
Spraing with a winter oil

It's not on a heater no.

Shall I take it out the pot and have a look? Or just change the entire soil?

Thank you for your quick replies.

Is It right what i see that the tip of every branch are green?

I think they are green and then you will have to just wait for the tree to recover.
It will make lots of new shoots if they are green.

If the tree recover then i would repot it with new soil and maybe in a bigger container. Good luck:)

No just one branch is green. The rest is dead.

Some of the branches will survive. Not sure if every branch will survive. Time will tell.

What is the most common reason for the leaves dropping:
Not enough sun light.
The soil has dried completly out for a day or two.
Change in temperature. If the temperature drops below 16celsius. Then the tree will drop its leaves.
Change in location.

What you can do:
Wait and hope for the tree to survive. Maybe take him on holiday:)
Put it in the brightess and warmest room.
Make sure the soil dries out between watering but not completly out.

Are the leaves turning yellow/black before dropping or do they drop as green? Did they drop fast or slow?

UPDATE***

There is still leaves growing on the same little branch but the rest is still dead.

Would it be a good idea to trim some of the dead branches back in hope it may grow?