1. What android app can you recommend?
2. What settings should I use to get the best detail and info? palettes?
3. How do I use it to detect cold issues and how do I recognise there is a problem?
4. When I test it, do I set my home central heating to high/low/medium temperature?
1 - I have the iOS version, so I can't speak to Android, but I would suggest starting with the official Flir apps. The iOS apps seem pretty good and I have not looked for any 3rd party software.
2 - It is easy to switch from one palette to another for a given scene, so cycle through them and see which is most helpful for your particular conditions. The one I have found most useful is called "Iron"; it is patterned after black body radiation colors and is pretty intuitive. Dark "cool" colors are cooler/colder and bright "hot" colors are warmer/hotter.
3 - It will show you temperature variations within its field of view. You already know what some of them are but this will help quantify them - if outside temperatures are lower than inside, window glass should be colder than surrounding insulated walls, air gaps will be even colder, etc. It reveals hot spots or cold spots you might not otherwise know about, such as might be caused by gaps in wall insulation triggered by sagging.
4. I would have everything set the way you normally keep it so you can see the effects of outside temperatures on your indoor living space. You can also do a survey from outside to investigate heat loss through windows/doors/vents/joints/weather stripping, etc. Ditto with summer cooling if you use it.
One thing that can confuse the issue when surveying an area that is larger than the field of view is the default setting that causes it to automatically recalibrate every few minutes. It rescales the temperature/color correlation based on maximum/minimum temps within the field of view, which can make colors inconsistent for a given temperature. This can be turned off in one of the settings, and the color spectrum can be locked in so you can compare photos taken at different times or of different views. This matters when surveying a room that contains a cold/hot source like an exterior window and then reframing the scene so it doesn't contain that source. The color to temperature correlation shifts if it recalibrates.
My biggest problem with the Flir One is the very small battery. I don't have much problem with short run time as much as the frustration of the battery being discharged about every time I pick it up. I find this limits its spontaneous use, since I almost always have to charge it before I can use it. That's ok for planned surveys, but it limits "I wonder what..." use.