I woke up during the night and I must have been thinking of this (I am prone to this I always seem to fix programming problems in my sleep
So I closed CumulusMX edited the .ini file to reflect the IP address and MAC address of the HP2551 and restarted CumulusMX. No such luck, couldn't find station, invalid checksum etc. etc.
Should this work or are the GW1100 and the HP2551 running different outputs even though they look very alike in their setup?
Just in case anyone wants to look here is the MXdiags.