Right, downloaded the 7.1 files again, reformatted the SD Card to FAT32, copied files to root and followed the update procedure as outlined to the letter. Status bar with oscillating multi-coloured rings appeared, ran through installation phase to Erase and screen went black. Unplugged HDMI and re-inserted without disconnecting power at any point and VOILA! We have 7.1 installed
Thank you ever so kindly. Always a pleasure to eat humble pie when my views are changed by solid support.