Even new batteries can fail.
As stated check all the battery and chassis/engine grounding points are good.
Try swap over the Starter Relay with an identical one next to it.
Did you measure the drain on the battery when turned off by using an Amp meter in series with the battery?
Any funnies on a 2006 CRD merits having a look at the wiring harness behind the Fuel Filter Head Mounting Bracket...the LHD 2006 CRD has a known problem that the Bracket can cut into the wiring harness going behind the bracket and being shorted to chassis.
