Sorry, Greenman, I missed your last reply somehow. I just decided to check back and ask if my last post had unintentionally put you off, and found that you had already replied.
These threads disappear down the list so quickly, especially in this introductions forum.

When I looked into this, I think I just 'assumed' that determination of solar midnight was possible, I didn't look into the practicalities of how it would be done. Your description was illuminating. It seems that my assumption was valid, but I hadn't realised all that went into the calculation.
Do you have any thoughts on determining longitude at sea?
Maybe we should go PM with this? I'm not sure that anyone else is interested.