Hi there, @LouA.
We've just had a go at this ourselves and it appears this option doesn't exist, we will pass this on to our Research & Development Team to see if this something they can add.
However, we have come up with a possible workaround you could try in the meantime.
If you turn on your timer and choose the option to Take 3 Photos. Then also turn on the Motion Photo option (also located in the Camera settings). This will take 3 short video clips after the timer has ended. This gives you the option to pause the video and 'Capture' specific frames. We understand this isn't exactly ideal, but it's an option.
We'll send back a message once we have some info from our Development Team.
Welcome to the Community! :smileyhappy: