Similar problem here, after a few weeks of minimal use from new, the phone reports memory full. Finally did a full re-set, losing all data. The phone was ok for another couple of weeks, then the memory was full again. I've demanded a new phone, but all I'm offered is a repair. If I wanted a recon phone I'd have bought one. Not impressed after 9 years trouble free service from Three.
You may be onto something. Only 1Gb memory? I have downloaded hardly any apps, and taken only a dozen or so photos. How can I transfer to storage, there doesn't seem to be a file system, or any control over storage transfer. Already called 789, no hope. They say it's a known fault, and keep making me wipe all my data. They've offered a repair, but I'm sure they'll also just do a factory reset.
You could also try doing a "clear data" on the "Google Play services" app. Or uninstall it. Restart the phone and and let the Google Play services rebuild or re-install. It's notorious for going wrong and causing memory issues such as you are experiencing.
Goto Settings>Apps and then scroll down to find Google Play services.