Q.Can I save my begonia?
I have a beautiful begonia (see pic) which flowered all through the summer up to mid-November.
I think it’s dying – nearly all the stems are limp and the plant seems to have collapsed.
I’m in the British Isles, house is cool 12 -15 Celsius during the day.
How can I save this plant please?

Certified GKH Gardening Expert
That is a beautiful begonia. Was it exposed to freezing temperatures? Your description of the plant sounds like what begonias look like after a hard freeze. If not a freeze, your house is rather cool for a begonia. They like about 70 degrees F (21 C) during the day and 60 degrees F (16 C) at night. Consistently cool temperatures can weaken or kill a begonia. If that is the case, you won't be able to revive it. The other possibility could be root rot. If that is the case, sometimes the plant can be saved by carefully cutting away the damaged roots and repotting in fresh potting soil. Here is more:
https://www.gardeningknowhow.com/plant-problems/disease/treating-root-rot-gardening-tips-for-housplants.htm
https://www.gardeningknowhow.com/ornamental/flowers/begonia/begonias.htm