Having investigated this further it appears that during the PPP session running at 230400 baud the HL7692 seems to start sending the following unsolicited results:
+CREG: 2
+CGREG: 2
+CEREG: 2
+CREG: 1
+CGREG :1
+CEGREG :1
+SIM: 1
+KSUP: 0
+PBREADY
It’s like it has reset itself during the PPP session - watchdogged?