This is a great flat. It's really well equipped and in a good location with historic Sintra easily walkable - the view is nice too. There's a supermarket and bar nearby and if you're arriving on the train we'd recommend getting of at the Protela de Sintra station as it's a shorter walk.
Rui and Anna are excellent hosts, very communicative and helpful. Whilst they weren't in the country when we stayed there they arranged for a local friend to help us when needed and were still very responsive to our questions. There's lots of nice touches to the flat and all round we enjoyed staying there.
It's worth noting that the noise from other flats can be a little disruptive. Whilst mostly they had quietened down by 9:30 if you're a light sleeper then I definitely recommend taking some ear plugs along.