I've had a lot of problems over the last many months with this fault code, see here:
http://www.volvoforums.org.uk/showthread.php?t=163422
Well, the saga finally ends!
After much investigation and replacement of parts that looked or tested as marginal, including a new intercooler - estimated £50 test v £120 replacement seemed a waste of money so just replaced it - and a full EGR clean without improvement, I resorted to applying logic and started looking outside of the normal ECM6805 causes listed in VIDA.
Turns out that the new exhaust front section 1st CAT had come adrift internally and was restricting exhaust flow causing poor flow at the turbo and low boost. Luckily I had retained the old, fractured exhaust so I had the old parts re-welded by a local fab shop for £40 (they wern't corroded, just fractured, but that's another Kwik-Fit useless parts story!) and substituted the old for new.
BINGO! The beast is back!
When all else fails, check the improbable. Perhaps the exhaust condition should be added to the list of ECM6805 checks?
Many thanks for all the help from the forum members over the last many months.
Steve