After much searching and piecing together various snippets of information from various internet sources I have now found a solution to this proximity issue.
See this reply to my post on the xdadevelopers forum regarding this which refers to a new custom ROM which has a fix for this applied.
So basically this fix can be applied to any ROM whether it be custom or stock and consists of a script which is run at at startup. The only criteria is that the ROM must be rooted. I'm afraid I can't offer any advice on this as I rooted mine a long time ago and I can't remember what method I used. However, if you check out the 'Jiayu G Series' forum on Modaco you will find advice regarding this.
If you are running a custom ROM which has init.d support then the script can be placed in the init.d folder and will automatically be run as the phone boots up. However, if you are running a stock rom as I am then it will not have init.d support and so the script will have to be run another way. What I have done is to use a free android app called SManager (link at bottom of post) which has the function of running scripts at boot time.
Create a file with the name '99proximity' and the following contents: (copy & paste from this box)
echo 900 > /sys/bus/platform/drivers/als_ps/psthd
Copy this file onto the SD card in the phone. I suggest putting it in a folder called 'scripts' rather than just loose on the SD card.
I have also attached the script file to this post to save you creating it yourself.
Then install SManager and then proceed as follows:
Launch SManager and navigate to your 'scripts' folder (or wherever you put the script file):
...Open the scripts folder where you will see the script you created earlier:
...select the script which will bring up the following screen:
when you first enter this screen all the option buttons (Fav, Su, Boot, Net, Blth, Wlck & Ntfy) will be greyed out as they are not yet selected.
Press 'Su' and 'Boot' which will then show up as activated as in the screenshot. These options specify running the script as root at boot time.
Press save, exit SManager and then reboot the phone Hopefully when the phone has booted up the promity sensor will work properly.
This fix has been working on my phone for a few months now so I know it works. Hopefully this tutorial is complete but it is mainly from memory so I may have missed something. If you do have any problems post back here and I will do my best to help.
Link to SManager on Google Play Store - https://play.google....s.scriptmanager