Thanks for the responses. I thought tomatoes were annuals. I see I was
wrong about that which goes pretty much all the way to addressing my
questions.

To answer a bit more, the room has an east facing window from floor to
ceiling so it's rather light but not light like a greenhouse. OTOH,
being as we're at a lower latitude than the UK (35 degrees north), we
get more sun than anywhere in the UK during winter.

The conditions were very very dry. I doubt my wife watered the plant
over winter. Our relative humidity is roughly 20% or less during the
winter time.

The plant is merrily producing fruit right now. We harvested our first
fruits from it yesterday.

Again, thanks to all for your responses and information.