Have a look at the Outdoor Action website: £259. It might also be worth looking at their Kirkwood range (pre-launch name for the Killington), which is very similar to the Diablo, but with flysheet-first pitching.
I know there are layout/pitching differences between the diablo 600 and the kirkwood 600, but is that all ? Will the kirkwood have the same saggy bedroom syndrome ?
I'm sorely tempted to change my diablo 900 for the kirkwood 600...?
I would take a look here before making any descision about the Kirkwood. Apparently there is no full inner, the outer is one piece with a sewn in groundsheet. The bedroom pods hang from the outer, so the remainder of the living space is a single skin rather like the Vango Colorado. The Diablo is a true double skin tent in that the inner includes the groundsheet and the outer forms a complete cover over the inner. This makes the Diablo quite a warm tent a you get insulation from both skins.
I've a 'new' Diablo 900 you can have for £200, only pitched once in the garden, I'm in Suffolk, email me at nicholasallan1969 at hotmail.com if interested