OK, just an update. I decided it was probably worth giving the unit a factory reset to see if that would make any difference. After doing that I restored the settings from a backup and found that there was no change. I then reset to factory default a second time but this time I re-entered all of my settings manually. So far from what I can see it’s all working. If I reboot the unit it seems it will always make a weather call and at this stage that always seems to be successful. I will monitor it for a few days and see how it goes.
Importantly, I should point out that before doing a factory reset I noted that I was actually having the same issue with the Zimmerman method. On further testing I suspect that caching of my web browser (Google Chrome) is likely to have distorted my findings because unless I force a re-read of the page, I can’t always see updates. I first discovered this when trying to manually enter settings. After saving my changes I would go back and look at them and find that they appeared not to have changed but after forcing a refresh, the setting had in fact changed.