Hmm, I had a brief bit of time just now but only enough to move a bit of the carpet. No nails in sight and my plan to wiggle something between the boards hit a snag - there appears to be some kind of filler between the floorboards. Still, found one that was slightly looser and had a dig!

Found two joists. Each was 65mm wide. Distance between closest faces was 340mm, so centre to centre would be 405mm. What I'm wondering is that, because the measurements were taken close to the stairs, whether this is representative of the rest of the room or whether the gaps are different because of the stairs. Somehow. Or maybe not.

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Measurement was taken about a foot away from the wall, roughly in line with the middle of the radiator.

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The stairs, viewed from the uncovered part of the floor. The joists I found would be roughly in line with this.

I don't know if the gaps might be different further out to the middle of the floor but didn't have enough time to check. An awful lot of moving might be needed!

The plot thickens.

Edited to add: I plan to check the depth of the joist but I'll need to find something thin and bendy first. I guess the depth is a more critical bit of information.