I've just had a look at your very comprehensive review Spandit, it's a bargain alright but at 4.5 kilos it's heavier than I would like it to be. There are some new defence 4's selling for between 70 and 100 pound which I think is a good price.
So you'd pay the same money for the same bag but miss out on the Tropen, which is a very compact summer bag? The weight I quoted was for both bags together (in stuff sack) - the Defence 4 is going to be the same weight regardless and once you've slept in it once, it's second hand anyway...