Hello again Mark,
Now, this is weird. Welcome, but weird.
I've just reinstalled Windows, fully expecting to have to restore GRUB, and hoping it wouldn't bite me on the backside with some mysterious error that would take up time I can't really spare..
However, the dual-boot still works. GRUB gives me the usual options as though nothing happened.
Thanks for your help, as usual. Next step is to upgrade Ubuntu...
I'll mark this solved now.