ecu on a td5 needs a strong voltage to start, if your battery is no good your only gettin a percentage of the jumping battery, so probably not enough, get a big new battery with a high cca rating, that should do it
It could be as simple as a £10 set of solenoid contacts off EBay. My 2000MY TD5 failed to start a few times so I fixed the solenoid contacts and never had a problem since. It seemed as if the battery, starter motor, engine didn't want to work but it was much less of a probem. Simple job as long as you have the correct tools as suggested by many forums. Just do a search on this problem and hey presto you can be back in business in a couple of hours.
You don't need to remove the starter motor just know where the nuts are and what sizes and what size socket you need.