NavList:

Dave P., you wrote:
"Whether we can live with five minutes uncertainty depends on your latitude and the Zn of the PLs. Personally, I’d complete two plots, one for each timepiece, expecting to see PL’s differing by 77 minutes of longitude E-W. I’d use the plot nearest my DR position (assuming there’d been no upheavals to the compass or log) , but always keep in mind that the alternate plot might be correct."

That sounds like the way to go. Assuming whatever event took out our electronics has not left us with too much time since the last fix, then if we work up a celestial fix quickly, there will be no doubt. Even if a day has passed, it's unlikely that we would have trouble picking the one closer to the best estimate DR position. One of the things I like to point out to lunarians sometimes is that a DR longitude is itself a measure of GMT. If the DR is "fresh" and we trust it, then we know GMT within some reasonable error range. And that alone can be used to get the error on a clock that's set wrong.

And Greg's idea is the next best thing: plot a Moon LOP with each of those fixes (by the two watches). The one where the Moon LOP passes closest to the fix from the other LOPs is the plot with the correct GMT. Of course if there's anyone with time to play around with actual "lunars" (measuring the lunar distance arc as opposed to plotting a normal Moon line of position), then that would be the most sensitive way of deciding between the two watches. If, as was the case with the photo at Greenwich, the difference in time is five minutes, that's definitely decidable with lunars, even for a beginner. By contrast the forty second difference in the follow-up photo would probably not be decidable except for a navigator with some skill and experience with lunars.

You also wrote:
"You have to admit. 10:00 BST was a good guess without looking at Richards watch."

Oh hell yeah! Judging only from the shadows on the buildings and the approximate season in that small photo (small as displayed), I could guess "sometime in the morning" but within five minutes?! That's pretty good.

Frank Reed