Hi, it's me back again
I've spent a lot of time chatting to Apple Support and they have offered to replace the phone and someone is due to be calling today about it.
However I'm still not convinced it's the phone as it didn't happen for the first four or five weeks that I had the handset. I downloaded the update to 8.1.1 the day it started happening. However, the only other thing that has changed is that O2 upgraded my contract to include 4G, so I'm now wondering whether that could be the problem, as that happened the same time as the iOS upgrade.
I did some testing by using a cheap payg sim (which doesn't have 4G) and the issue didn't happen, so I've turned off the 4G option, put my original sim back and will see if it happens now.
It's not a complete solution because I DO want to be able to use 4G but at least narrows down the problem if that is the case... only time will tell. I'll keep you posted.