How can this ACARS message be decoded?
ACARS encoding is usually different from customer to customer.  Decoding it can be very difficult, even if you have access to the aircraft to have some idea of what is being sent.  Usually, the manufacturer won't share the details with anyone other than the original customer, expecting subsequent operators to pay for custom coding on the ACARS.  A general introduction to the basic standard codes can be found here; www.universal-radio.com/catalog/decoders/acarsweb.pdf .   The 'out return in' messages are pretty much standard.  No doubt there are enthusiasts out there who have decoded at least parts of the carrier specific/aircraft specific coding.  I worked with an ACARS equipped fleet where most of the messages sent were carrier specific and I never did manage to get the decode of the top of climb (TOC), cruise (CRZ) and top of descent (TOD) reports that were being sent automatically, despite the co-operation of the vender and the original customer, as the original customer had thrown that information away.

LIkely decode from the data you have supplied;

POSN70071W097007 = Postion North 70 degrees 07.1 minutes, West 9 degrees 00.7 minutes.

Sorry, it isn't much.  G-INFO isn't the registration either, it would seem.  Registration information is usually hard encoded in a custom plug for each aircraft's ACARS system.