OK, I believe I've solved this, I've had no issue for almost a week now.
I believe the issue was to do with Dobly.
On my LG sound settings I changed audio mode from auto to Dobly manually. Since then I've not had a signal lost image since resuming from stand by.
There must be something going on during the handshake, it's probably my TV not liking switching from pcm sound to Dobly, but it seems happy to be dedicated Dobly.