Depends slightly on the age of the Defender and how fussy you are about the great unwashed on your doorstep.
I’ve just sold a long in the tooth Discovery 3, with 212k on the clock via Autotrader - it sold within 3 hours of going on there, selling to the second caller but you need to decide if you can cope with chancers who may well want to look at your security and then nick the thing later at their leisure... cynical but possibly true...
If it’s a nicer Defender, try Defender2.net - again, you have to have a post count to sell, but Martin will let you buy an advert if you don’t have the requisite participation on the site. Good folks on there and probably worth taking a look - I’d suggest you’re less likely to get the idiot questions - I used the forum (def2) to sell my father-in-law’s 2002 TD5 some time ago.