chainsaw,
You have a choice of two ECUs either MSB 000080 this one would have been the original part fitted to a MY2000 Defender TD5, it is the early type you will have to program the AS10 alarm to the replacement ECU and set the five injector values, the fuel map is not programmable on this model ECU without changing the chip.
The other choice is NNN500020 this is the later replacement ECU which would be supplied as a replacement by a main dealer, again you will need to program the AS10 alarm to the replacement ECU and set the five injector values, also program an appropriate fuel map because unless it has come off a 10p Defender an incorrect fuel map will be installed.
As an added note I do not believe either of these ECUs are available new the last price from LR for NNN500020 was around £700.00. Used price for MSB 00080 expect to pay £125.00 + and the prices for NNN500020 are going through the roof I have seen used, condition unknown advertised at £500.00.