Apologies for the late update but I've been sidtracked on other projects!
Got this working by reading the manual properly and not bloody skim reading it!
Bare in mind this is a tenanted system, so i can't have everything going to the Embedded AA, so...
DDI > NTHG (#1000) CReRA to Voicemail HG.
Menu node mailbox setup as 1100 (as it ignores the #) - and you must set the extension as the same as the mbx number! Important!
Dial into the NTHG and when the prompt plays, dial the extension you want.
EDIT - set the embedded system to allow dialling of any extension, with your exclusions for security.
Thanks again.