Update: I fitted it on Saturday morning with no major issues.
Worst part of the installation was cutting the hole in the lathe-and-plaster ceiling, I eventually found my jigsaw worked the best, although it was awkward to use upside down. Tip for others, the template they give you to cut around is too big by about half an inch, not leaving enough lip to drill the securing screws into... I ended up attaching battens above the hole in the loft and using longer screws into them.
I initially set the Drimaster to fan-speed 5, just to blow the cobwebs away and get things moving. It was bloomin cold so I soon turned it down to 3!
By Sunday morning, most of the upstairs windows were much dryer than usual, with just the worst room (daughter's bedroom) still having a fair bit of water on, but even that was better than usual.
Downstairs still had a fair bit of water on the windows, but they were also better than usual.
Today, even the window in my daughter's room only had an inch-or-so of condensation, and the downstairs had further improved. The dehumidifier that we usually run over-night hadn't switched on, presumably its humidity sensor thought the air wasn't damp enough for it to be needed!
I'm hoping for the last of the condensation to have gone tomorrow and I might knock it down to setting 2.
Yippee! After 16 years, a dry house!