Thanks everyone, for the quick replies and for the welcome!
To answer a few points, I've had this ivy for 4 years now, and it has been in the same place that whole time - and flourished to the point of taking over half a fireplace (don't worry, the fireplace is not in use!). The fact that it has done so well over the years is part of the reason that I'm so keen to save it.
- Watering: around once a week, though I will start easing off now as we head into winter
- Orientation: nearest window faces North-West, although given that it has done so well over the years, presumably that isn't a problem.
- Temperature: I'm not home much during the week, so the house is unheated. Outdoor temperature is around 13/14 C right now (in Scotland)
- Humidity: I don't have a measure, but I do have a few tropical plants which I mist - not sure if that will impact things?
- Repotting: around 5 months ago, and it has flourished since then
My concern is that this ivy is showing the initial signs of something passed on by the friend's ivy (fungal infection?). My hope is that I'm just noticing signs of mistreatment on mine (temperature? over watering?) because I'm watching it more closely after the death of the young plant, and that it won't come to much.