Thanks to all the help and advice from people here, I managed to fix the problem with the first stage of my sectional-concrete-garage-into-workshop conversion (the problem being that a secondhand uPVC window mysteriously grew overnight and proved too big to fit into the wooden frame I'd built for it).
In case anyone else is brave enough or daft enough to contemplate doing something similar, I thought I'd offer three pictures:
The garage as it was:
formatting link
formatting link
with window and door installed:
formatting link
structure survived having several of the panels removed (and staying that way for several days), though I did bolt the single freestanding panel that remained between the door aperture and the window space down into the base (the rest of the garage is just resting there, held by its own weight). Even so, there was definitely a degree of movement until the metal door frame and timber window framing went in, and still some slight settling after that.
Next stage is fitting another secondhand window into a wooden frame in the end wall. Measure twice, cut once, measure twice, cut once, measure twice, cut once...
Bert