As the autumn progressed and it started to get colder I began to think about bringing it indoors as the greenhouse was unheated and the growing instructions suggested that the plant wouldn't like temperatures below about 5°C.  Once we started to get overnight temperatures close to that I brought it in an put in the lounge where it would still get good light.

Pretty soon though it started to shed the odd leaf.  There seems to many reasons why a lemon can start dropping leaves - too much water, not enough water, too much feeding, not enough feeding, etc.  In other words there seems to be more ways to make a lemon drop leaves than there is to make them stay on, and despite trying changes to feeding, watering position and light pruning it just kept dropping leaves until it was completely bare except for one half grown fruit.  My wife was convinced it was dead although I didn't believe that since there was still green wood present and that one fruit was still looking healthy.  Some branch tips had clearly died off so I snipped those bits off.  

Incidentally, it's only once a lemon has dropped a few leaves that you realise how viciously thorny the stems are!

Come spring time this year, I thought I could see the first signs of new leaf buds on the stems so as soon as the greenhouse had warmed up a bit, I moved it back out there, still with that one fruit clinging on.  Sure enough it started producing healthy new leaf stems and continued to do so all summer (which has been unusually consistently warm and relatively dry), but there were no flowers at all this year, and that one lemon is still there - maybe a little larger but still green rather than yellow.

I've read some other advice that recommends a minimum 10°C for lemons so this autumn I've brought it indoors much earlier, since maybe I pushed my luck too much last year.  So far it still looks pretty healthy with no sign of wanting to drop leaves (fingers crossed).

However, I am wondering if maybe the garden centre's suppliers "force" the plants to produce flower and fruit to look attractive on the displays and encourage sales and the poor purchaser then has to put up with a couple of lean years until the plant recovers some equilibrium.

We'll see how it fares over this autumn and winter.