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: http://tinyurl.com/3zm4f25

Inbetween: http://tinyurl.com/3aut2o3

And with window and door installed: http://tinyurl.com/3t6bbtc

The 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