**Solved**
Ok, so running H323 over a VPN requires the disabling of both SIP ALG and H323 ALG
ALG Status :
DNS : Enabled
FTP : Enabled
H323 : Disabled
MGCP : Enabled
MSRPC : Disabled
PPTP : Enabled
RSH : Disabled
RTSP : Disabled
SCCP : Disabled
SIP : Disabled
SQL : Disabled
SUNRPC : Enabled
TALK : Enabled
TFTP : Enabled
IKE-ESP : Enabled
Then I created a security policy as per the assitance on this thread for junos-h323 only (And then another policy for application 'any'
I now have RTP in both directions. Thanks for everyones input.