Only owned our 2016 SE CVT for a few weeks but we have recently driven from Tyneside to Cornwall, Devon, Dorset and back - about 8 days and 1200+ miles - averaged 54.9 mpg (trip computer) which seems OK assuming that it's reasonably accurate.
Our previous diesel car, a Hyundai IX20 1.4 CRDi, based on previous experience, would have probably done the same trip at around 56 mpg, so I'm quite impressed with the petrol Jazz.