Cheers Guys. My Garmin Forerunner recorded 47.66. It did lose the signal in Whitchurch woods at one point and I was a bit slow restarting it after one of the checkpoints.
Back ache as well, but am putting it down to running (more like trotting) the whole race in my Inov-8 305 trail shoes, when if I'd known what the course surface was going to be like from CP4, would have changed to my road shoes.