Hi, hope someone can help. Not sure if I've over watered, over fed or too much direct sun.
Plant was bought 7 months ago, it had been wired in the past as there were rusty cut marks in the branches and I found some rusty wire embedded in one part.
New leaves form and then start to turn black pretty quickly. Not sure what I've done wrong but obviously something. Any help would really be appreciated.
Thanks to all in advance.

If you have kept it inside that is the problem. This is a hardy species and cannot surive inside. The wire should not be rusty, so the wire used was unsuitable for wiring, it should be aluminium. Watering should be when needed not to a schedule, so when the soil is almost dry then water thoroughly. As far as leaves are concerned there could be aphids causing it.
Mine is in full sun all year.

Thanks for the advice. As with all mass produced bonsai they don't care about the wire they use to train them. Is it ok to keep In a gazebo with open sides? What about winter as I'm sure they won't like a frost?
Have you any recommendations for aphid treatment and how can I tell if there are aphids present?
Once again thanks for your help.

The black on the leaves a sure sign of aphds. They are hardy and do not need protection from frost. Use an insecticide.