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!
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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.