The majority of fat free yogurts are free on Slimming World, even supermarket own brand. You have to be careful if it doesn't specify 'fat free' and instead says 'low fat' or 'less than 1% fat' - those ones could be synned. EG: Morrisons fat free yogurts are good, but Sainsbury's less than 1% yogurts have a syn or two.
Also, Activia FAT FREE yogurts are completely free, be careful of the labels though, as the packaging for the synned Activias is identical to the fat free ones, the only difference is the words 'fat free'
Hope that's helpful! xx