Wow....thanks for all the answers. I really didn't know that modern aircraft required such fine-tuning in terms of weight distribution.
I didn't complain or anything as it was highly unlikely to get me anywhere in the circumstances, just politely asked the stewardess. It did feel at the time, somewhat irrationally I know, that I was being singled out in some way!

One thing that confuses me though, is that when on a flight with a lot of free seats I've seen a total free-for-all once the doors are shut. I've seen the cabin crew actually encourage this. If weight distribution can be such a fine thing how is this allowed?