Thanks Roger++. That sorts me out.

My problem, which is perfectly clear even from the ascii art, but eluded me last night is that my assumption that ...the pentagon, with the five sides the same width as your strip.... is completely falacious. The strip passes through the sides of the pentagon at an angle (of 18°), hence the width is just under 5% narrower than the length of the sides. Seemingly a small difference when the strip is only 10 mm wide, but multipled 48 times has a dramatic affect on the overall length of the strip.

That explains why I was having trouble tying the dratted thing. I was 24mm short!