Right, after a couple of hours mucking around, I think I've worked out this out... (this is for iPhone - I can't speak for Android!)
The answer lies with Lightroom CC Mobile.
I'll try and be as succinct as I can be but please ask any questions you like.
1. Firstly you must subscribe to the CC package but to always have the most up to date version of LR and PS for only £8 a month is frankly very worth it. And with it comes LR CC Mobile.
2. LR Mobile only syncs with one catalog at a time in LR. As a professional photographer, I start a new catalog at the start of every calendar year anyway - not only does this keep things small but also fast. It also means if I lose a catalog, I'm only losing one year's worth of photography - not everything I've ever done! So, for LR Mobile to work, I've created a separate catalog on my computer, and I've put it (unlike the others) into Dropbox so it syncs across my various laptops and desktops (although you don't have to do this!). I've called it 'Overarching iPhone Images'. This also useful as my iPhone photos span back over 8 years and I like to have them separate from my DSLR shots in LR so it all works neatly and I don't get muddled.
3. Download the LR iPhone app to your phone and log in using your Adobe login. In the settings, switch ON the 'Auto Add Photos' option. Then in the app, select 'Add Photos' and add a few images from your native IOS Camera Roll as a tester. Into a folder called LR Photos on your phone, those photos should now appear.
4. On your computer, open LR in the new catalog you've just created for your iPhone photos. Scroll over your name in the Identify Plate and a little down arrow appears. Click on that and the first open is 'Sync with LR mobile' - switch it on... Hopefully, depending on your internet connection, the couple of photos you selected will sync automatically into your new iPhone catalog allowing you to then use LR in the normal fashion on your iPhone shots.
5. Because you switched on 'Auto Sync' every photo that now appears in your camera roll will then end up on your computer (including ones from WhatsApp) - BUT ONLY AS LONG AS THE LR APP IS OPEN ON YOUR PHONE!!! However, it will not sync everything historic. You have to do that manually by 'Selecting All' in the app and then adding it to the catalog... Mine is currently uploading 13,000 photos!!!
6. I've also downloaded the app onto my iPad and it's even better there. However, I've not yet worked out the implications of how much space this is all going to take up so keep an eye on that. However, THANK YOU ADOBE!! You have saved me untold pain and a real sticking point... All my iPhone images will automatically sync into LR on my computer and, even better than that, in a different catalog so it's all nice and tidy!!
7. Working in reverse (and I've not tested this yet) to get photos from LR on your computer to your phone, I suspect that you could export the folder of finished shots from one catalog, import it into the iPhone catalog and the sync process will just automatically happen. Further to that, I suspect (but not tried) that you'll then be able to save to your Camera Roll, and thus back into iCloud and across your devices... That might be a bit long winded but it would certainly avoid having to find a plugin to save directly from LR to your Camera Roll... Just a thought.
Anyway, I think that's it. Good luck and if I hit a road bump I'll come back and let you know... Any DM questions, do email me - enquiries(at)elloydowen(dot)com and I'll try and help you.
Cheers - Ed
www.elloydowen.com
Chelsea // London