Usual guideline is that they don't NIP you if you're within 10%+2, so you should be OK in a 70 up to 79. That's true speed of course, not indicated, so on the speedo it might read 83ish? Some Police forces will offer a Speed Awareness Course for recorded speeds up to 10%+9, so in a 70 that's 86, which might even show 90 on the speedo. On a SAC you pay more or less the same fine, don't get points, but are patronised for about 6 hours. How do I know this..... :-(