Okay, so here's the thing... I was writing "book #10" before Christmas and had got probably, ooh, 80% of the first draft done before I shifted to editing "Invasion".
"Invasion" is now launched and sailing off into the big, bad world, so I went back to "book #10" to finish off the first draft and start on the edits.
"Book #10" is a new departure for me. It's a time-travel book, with some of it set in 1762. Before I started the edits, I wanted to check a couple of things with historians. I was checking two fairly general things - one about the construction of an outbuilding; the other about whether a soldier barracked at Fort George would be able to spend much time with his brother in Ardersier. In the grand scheme of things, these seemed insignificant and innocuous and I was checking them because I wanted to ensure the 1762 bit was authentic.
They genuinely were pretty minor things, and the historians were full of lots of fabulous information that helped enormously both with them and with various other things. However... it turned out that the brother wouldn't have been stationed at Fort George if he was in the army. That information, plus some general chat about something else entirely has blown two enormous holes in the plot!
Ah well. Just goes to show that it's always good to talk to experts (even if they do end up demolishing your plot before your eyes).