It will have been worth about £28,000 new yes. If you take a look on the Land Rover website, it says that the Defender starts at £19,740. Thats the price for the very basic, out of the factory Land Rover Defender 90 Pick up. It has no fancy toys, white steel wheels and basic tyres. What you have is a few models up. You have a Land Rover Defender 90 Station Wagon, so you have a roof at the back, side windows, alpine windows (in the roof which are also an additional extra), rear seats, alloy wheels, decent tyres, a snorkel and a roof rack. So the 'new' value of your Land Rover will be something similar to what the salesman has told you. Land Rovers don't really tend to loose their value. What they do however, is drop to a kind of 'level'. For the TD5 you're talking between the figures I suggested earlier and then the better examples of the TD5 (like yours) will fetch the higher end of the scale, where as the abused, mal-serviced examples will fetch the lower end of the scale.
With regards to economy, you should be able to get around 30mpg out of the TD5 providing you drive it properly. Inner city driving will decrease economy to around 25mpg, where as a good motor-way run will see it doing about 35mpg. So you can start to worry about your fuel costs a little bit too now! Generally speaking though, they're not that bad on fuel.
-Pos