I've got an Osmo+ with the dreaded black screen issue. Tried every single thing I can find online from firmware, changing SD card, reset wifi, all with several different devices both IOS and Android.

Has anyone EVER found a solution other than returning for repair?

If I have to return it (I am in the UK) is it a high cost repair? Is it worth it?

If DJI were UK based they would have to repair / replace for free as it is clearly an inherent fault and well documented by many people.

Shame such an expensive bit of kit is just so unreliable and you're lucky if a piece of DJI kit works flawlessly and as advertised.....

Hi S.Leah, sorry for what happened to Osmo+, was the gimbal able to be controlled? If yes, please try to wipe the PIN contact with 95% alcohol and cotton swab. Let the alcohol air dry and try again.  
If it is still not working, please force your Osmo+’s firmware to update:       
Download the firmware to the root directory of SD card;
Before power on, keep pressing the Trigger and Record button at the same time, then press the Power switch for about 5 seconds until the indicator blinks red and green alternatively.
A few minutes later, when the indicator becomes solid green, delete the firmware in SD card and reboot the Osmo+.

If there is no luck, it is recommended to send back for the diagnosis, the quotation will be sent to you after the evaluation when the gimbal arrives in the facility. Please start a ticket here: https://repair.dji.com/repair/index?site=brandsite&from=nav