I would suggest you get the oil changed every 6000 miles, that is half the recommended interval,plus a general check over (air filter etc). The air filter is only a few pounds so you might as well change it every 6000 miles also, if using the Defender in dusty conditions.
The TD5 has a centrifugal oil filter as well as the canister so the oil tends to last longer. The canister oil filter can be changed every 12000 miles, but again it only costs a few pounds so it might as well be replaced with every oil change.
Make sure that good quality synthetic engine oil is used.