Has anyone any experience with FrSky receivers?

I don't know much about the Radiomaster radios but OpenTX is what it is. Don't worry about IDs the radio randomly creates these so they are unique. However, if you flash the RX you need to re-discover. From what you say, you have good RSSI, Telemetry good, etc. Is the RX new? Is it running on V2 firmware (last up date July 2020, V2.1.2)? Unfortunately there is no way of knowing except flashing it again. Vario should come up on Discovery with no issues. Also try re-discovering with TX further away from the RX as the signal might swamping the radio.

Did you buy the RX from T9Hobbysport?

I don't know which version of firmware the Rx is running but I bought both Tx and Rx from HobbyRC recently and as you point out it has good telemetry. The latest firmware update was in 2020 and there has been no updates to fix unrecognised sensors.

I have deleted and re-discovered the sensors while the Tx is distanced from the Rx to no avail. Unlike other receivers it doesn't seem to malfunction when close to the Tx.

Before I contact HobbyRC or send the receiver back I would like to try binding it with someone else's Tx to see if they have more success.

I don't know when I will be up the field but in the meantime ask John B to bind it to an empty model on his Taranis. That will prove it either way. I have only one of these G-series RX in a slope glider. I use independent Varios in the other models. Never any issues although Varios rely on barometrics so you don't get accurate altitude (weather conditions effect it). Vertical speed + or - appears to be okay for thermaling.

Yes we tried to do that but couldn't get it to bind. It would not recognise the Rx. We couldn't find FrSkyX2 LBT(EU) protocol.

Is John's TX using latest firmware? I would email Richard at T9 and he will ask you to send it back for testing. I have had only one issue with FRsky RXs and that was with an ACCESS R4 which would not register. Replaced with no problems

I sent the Rx back to HobbyRC and they promptly advised that the Vario had been switched off in the Rx. This is done by holding the Bind button for 3 seconds after it has bound, something I accidentally did without realising. The Rx is back with me and all is working well now.