I thought nothing of it until 2 days ago the green leaves started to shrivel (but not turn colour) like the plant was dehydrated - but I have been spraying the tree and the soil looked quite dry so I have watered it but to no avail.
This statement seems to imply that you had not watered the tree, other than spraying, for a month. Is this correct? This is not an appropriate approach to watering your bonsai. It needs to be watered immediately and thoroughly. I would probably water it by the immersion method ONCE as an emergency procedure.
From there out watering from above with a watering can is the best technique. The soil cannot be allowed to dry out again, nor should it be kept constantly wet. A cycle of thorough watering, then partial drying should be established.
This site has basic information that you should review, pay particular attention to the section on watering. [url]https://www.bonsai4me.com/Basics.html[/url]
Check this out as well. [url]https://www.evergreengardenworks.com/bonsaip.htm[/url]