Hi Def90
As Western says, there's lots of info in the two threads already started plus links to info elsewhere.
Also look here for info: TFL Website
If your Defender is registered as a utility/light 4x4 or commercial vehicle, you are stuffed unless it runs on petrol or has the costly filter fitted.
Options:
1. Fit the particulate filter to make it compliant;
2. Convert it to a v8/petrol;
3. Move outside the M25 (the LEZ covers most of the area inside the M25) and don't drive your landy back in;
4. Become Prime Minister and take us out of Europe.
It's a jip but there's not much else that can be done about it I'm afraid. Some people have tried for reclassification of their vehicle with the DVLA but I'm not really sure what difference this has made and if like me you have a hardtop and you don't want to change it, this won't be an option.
Regards,
John