Don't buy a sofa from M&S, seriously. Even if it appears to be a bargain. We bought 2 leather sofas from them a couple of years ago and when delivered one of them had a big hole in the back, which they hid by placing the sofa against a wall. When I discovered it a week later they said it was tough. I got Consumer Direct to write to them saying they couldn't do that, but still they said it was tough. So I sued them. The whole thing was a nightmare and they were so unbelievably rude and difficult. On that basis, you have had a lucky escape if you don't get the sofa.
(But to answer the question, no YANBU at all - they are tits.)