Thanks MVeagas21. This has been going on for a while, so not sure when it started or what the circumstances surrounding it were. The phone has not been dropped by me, but it is an Apple reconditioned phone. My initial problems appeared to be because of the 'live' setting and so I attributed everything to that. But now live has definitely been off for many months, including the setting that prevents reverting to live between uses.
I'm also having troubling replicating the problem at the moment. I looked back at a series of photos from last week and it took 2-3 normal photos, then 1-2 .mov files. It did various combinations of this over the course of about 25 photos in a single series, under the same conditions and with the same settings engaged. The .mov files are always 1 second.
Many thanks, appreciate your help!