EDIT: Sorry Steve. I guess I posted just as you were...lots of answers.
Hi Simon, I was a pastry chef for 25 years of my life so I may be able to answer some of your questions...if I haven't forgot everything by now...
The first thing is to get a good proven starter. In the absence of a friendly bakery nearby, a friend with a good starter, a traditional farm....it's best to purchase a kit from a reliable source. There are good ones available on line. Sounds like you have this part covered.
The second thing is that the starter needs to be well cared for to survive long. The longest I ever maintained a continuous batch was five years. It would have been longer except I went on vacation and the new Hotel Ex. Chef threw out my starter thinking it had gone sour...Well yea! That's the point. It's called "Sour Dough"
for a reason
So, be sure to keep in warm draft free area of the kitchen, feed twice a day with equal part by weight of water and flour. Less than twice a day it looses vitality and will become useless. Each feeding should be enough to double the size of your starter. Before feeding remove half of the starter and use it...for pancakes, biscuits, even cake, give it to neighbours for their own starter...or pitch it. Other wise you will have sourdough enough for your entire city within a couple weeks.
I suspect that your friends issue in the new house is not due to environment but more likely their routine has changed and their sourdough is not being fed enough or properly cared for.
To your questions: Do not add salt or anything else (no flavourings) to the starter (if we need a lot of starter in a big hurry we would add some sugar the day before, yeast loves sugar just like the rest of us). Salt is used in bread to control the growth of the yeast, you're not trying to control the growth of your yeasts but expand the growth so no salt in the starter. In fact any dry flavourings (salt, cracked pepper, spices, seeds, nuts...) should be mixed with the flour (dry ingredients) when making the loaf and any wet flavourings with the water or liquid you use in the loaf. A little trick I learned from an old German baker was to substitute up to 1/4 of the liquid (water usually) in your bread with buttermilk. This gives the yeast a little boost during the final proofing. I also used the buttermilk in pancakes 1/2 - 1/2 with milk to make creamy buttermilk sourdough pancakes....
OK, now I gotta go make some flapjacks for lunch.