Hi tony we just filled our ilr used the settlement checking service well worth it. They sorted out everything even photocopied all the docs and sent it on to the ukba. We understood the question (how long have you been living in a relationship?) as how long have you both lived together in the same house.
I would check they will not send your doc's to the ukba as if your wife visa ends on the 20th December ukba will not accept your application until 28 days before or you could lose your visa fee.