Hi Stan,

This could be one of several things:

• evergreen plants don't shed all their leaves in one go like deciduous ones, but are constantly losing them, a few at a time and it's perfectly natural.
• the temperature in the conservatory might be a bit too low, especially if the lemon is near the glass.
• the light level may be low during these winter days.
• the air around the lemon may be too dry, as lemons like high humidity. You can cure this by standing the pot on a wide saucer of pebbles filled with water. Don't sit the pot down in the water or the roots may rot, the idea is for the water to be able to evaporate up around the leaves and raise the humidity.

 As long as the plant looks healthy otherwise, then it could be just a reaction to our dull winter days, in which case, it'll pick up as we go into spring. I hope this helps, but do get back and let us know how it goes on, won't you?