The message
from "Dave" contains these words:

I've had a dozen lemon seedlings 'growing' in the greenhouse over winter
(North West). It's not heated, they're not propagated (just growing
out of a
seed tray) and the greenhouse is cetainly not frost free judging by the
amount of plants that have died in there over winter. I've also had one
growing inside the house. All of them have kept their leaves but that could
be down to their age. The cold doesn't seem to have had any affect on them.
I have very little gardening knowledge but could something else be causing
the loss of leaves?

No. Well, extremely unlikely.

I left a couple of young lemons out 'sacrificially' last autumn to see
if they'd survive.

They haven't.

I expect there are varieties which are hardier than others, though.

--
Rusty
Men love women, women love children, children love hamsters.
(Alice Thomas Ellis)