I found our accommodation for the trip by using my tried and tested Google maps technique: go on google maps, find national park, find town within national park that is accessible by train from London, search all self-catering accommodation within this town. Simple!
This time I was lucky enough to click upon Pine Bank Chalets in Aviemore which had a choice of self-catering log cabins. The one we stayed in was clean, spacious and most importantly very toasty warm.