After several phone calls, two full restores as new device I finally found my way to the local Apple store with a genius appointment. She read the notes, check the diagnostics which came back it was having issues. She did a DFU instal to see how I get on with it, if no change she was offering an exchange. But so far, six hours of usage and not one crash. Hammering Safari, 10 tabs open, lots of flicking about, scrolling etc. I do hope this has fixed it.