charlotte,
Welcome to the site. As you may know this species has the reputation of being difficult to manage but since this is a recent development your general care may not be the issue.
In the last couple of weeks leaves have started to go brown at the tips (mainly on one side of the tree), and fall off...It was positioned near a window, with a lamp to one side (the side which is now going brown) could the light/heat from the lamp have donw this?..The leaves on the other side of the tree seem ok (green and look healthy
This seems to be the most obvious thing to consider. Is the light a recent addition or have you perhaps moved it closer? Another thing to consider; has your central heating become more of an issue at the same time?
Incandescent bulbs are a poor choice for indoor growers. They produce too much heat in relation to the amount of energy they make available to the plant, not to mention that the wavelength is not ideal for general growing. Fluorescents are a better choice. The wavelengths, depending on the tubes you choose, are more appropriate. They can also be placed much closer to the plant without causing heat damage.
Most experienced growers avoid watering by immersion on a regular basis. In the short term there should be no problem but over time this favors the accumulation of salts/minerals that are present in fertilizer/water. By watering copiously from above you help to flush away these unwanted materials.
[url]https://extension.missouri.edu/explore/agguides/hort/g06515.htm[/url]
Norm