Thanks, rebooting seems to allow me to download another 4Gb of maps but it's worth noting that on reboot, the app incorrectly reports I have no downloaded maps anymore, but this isn't true as if I try to download an already downloaded map, the progress bar zips across the screen instantly so there is definitely a bug in the app. It also now reports I have 1Gb of free storage even though it's letting me download more than that.
Also, the 4Gb limit is not a problem with iOS, it's this app, its using a deprecated method for accessing the internal storage. Calimoto and the BMW Motorrad Connected app have no issue with reporting the right size of free internal storage.
The BMW app did, in fact it had almost the same issue as this app, but I raised the bug with them and they fixed it within a week.