GPG Signing based on recipient

[Mark Sansome <msansome@xxxxxxxxxxxxxxxxxxx>]

Hello all,

I have a number of different identities (bit schizophrenic really..) i.e. Home 
/ Work / Mailing List etc.

I would like to sign each mail with a gpg key appropriate to that identity. I 
have tried the following send-hook, but perhaps my understanding of the syntax 
of hooks is incorrect. Anyway it doesn't work:

# Default gpg signature for majority of mail
set pgp_sign_as=1234567891234567
# Identity and gpg signature to be used in all mails to "marktest@xxxxxxxxxxxx" 
send-hook '~C marktest@xxxxxxxxxxxx' 'my_hdr From: Test Account 
<test.account@xxxxxxxxxxxx>' 'set pgp_sign_as=98765432198765432'

Is it possible to do this?
Is my syntax incorrect? 

Thanks in advance...

Mark

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