Our maxi cosi isofix system works well as the front leg keeps the base supported so it doesn't dig into the seat.
Not sure what age the thing finishes at though.. I guess you may be looking for different seats.Edited by: Rednaxela
Irecommendthis ISOFIX seat:http://www.concord.de/en/#/products/driving/car-seats/transformer-pro/(the black+red looks good in my black+red up and it has won the ADAC award for best child seat). It sits firmly in place on the leather look seat when mounted correctly and does not move at all.I'm VERY happy with it as it is compact (no issues having it behind the driver with the driver seat pushed far back).
They say rear facing seats are much safer, but I really don't know in the case of the up! as the boot is shallow. I would think that in certain accidents (hit by truck from behind etc.) a forward facing could be safer. And it's certainly easier in use.