As promised, here is an update now my Forester XT is back from its first service. They road-tested the satnav and agreed with me that it was not working properly and would need to take the matter up with Subaru UK. Particularly obvious during the road-test were the verbal announcements happening as you are driving past the required turning!
Just now I had a follow-up call, to let me know there is a software update available. So the car is booked back in for the SD Card to be re-programmed and for this to be installed at the end of March. I will post a further update after then, when I hope to find the software update has cured the various malfunctions we have endured up to now.
Once the satnav is sorted, the XT will be an even better car to drive - verging on perfect in my opinion!