Thanks for your reply
I am now up and running with the G5. I have tried with both xDrip and the Dexcom iOS app, neither fill in the missing data when the transmitter has been out of range, so I have a G5.
The xDrip G5 Facebook group has quite a lot of talk about new G5 firmware that was released in Nov or Dec, This required a new release of xDrip with an extra setup parameters that needs setting for the new release, it appears that I need this set for communication to work, implying that I have what they refer to as the new firmware release. There is no mention there of back filling missing data nor the G5+.
My Transmitter has firmware 1.0.4.10