AirPods disconnecting from iPhone?

My AirPods have 100% battery and the case dose also, I connect them to my phone and then play music. Within 5 mins, the headphones have disconnected. Sometimes the left ear will connect after a few mins, but the only way to get them connecting to the iPhone again is to put them back in the case, leave them 15 seconds and they start working again. But then 3-5 mins later they will disconnect!

I've tried the AirPods on my MacBook Pro and with a friends iPad and they worked fine, without disconnecting. This must be an issue with iOS? I've noticed lots of other topics with the same issue. Resetting the AirPods do nothing. Any help appreciated!