The result wasn't a Forest Canopy - more of a Thicket or Copse. It isn't even big enough for a headscarf.
So, after much musing and head scratching and unladylike language, I measure my tension on the shawl and it was 28 sts x 38 rows to 4" - much, much smaller than my tension square!!
After more thought I realised that my tension on 4mm Addi lace circular needles must be different to tension on the 4mm straight needles I used for my tension square. I went back and did another tension square using the circs and it was very different. I then did a tension square on dpn's and, lo and behold, my tension was different again. I have now frogged the shawl and will start again making a tension square on circs this time.
The moral of this story is: I must always do my tension square on the actual needles I will be using as different types of needles give different results.
On a positive note though, I have finished my spindle spun Alpaca. It weighs 100g and measures 350 yards - maybe I should use it for another Forest Canopy??????