Hi there can someone confirm that a bmw e39 dsp amp will work in the l322 for me please or not ?

Hi Steve,
Yes - though the DSP amps are coded for the vehicles they are installed in, so a DSP amp from an E39 couple should not sound right in your RR. Though I've never heard of anyone complaining about this and I am quite sure that someone, somewhere has.

Make sure it is the same type of amp, above all that all connectors are exactly the same. BMW had several hifi systems available and people tend to call everything DSP... 
RRC 2Dr, RRC 4Dr,
P38, and 2 L322s
(wife thinks I'm nuts - prob right, too)