Thanks to all those who replied.
I now have it up and iterating.
Arthur summed it up best with his email which follows.
Dear Steve, you've fallen foul of a little trap.
In the model line you have
> fat start ~ Trait Tr|1.weight !r -weight.animal Tr.animal Tr|1.weight.animal,
> -weight.an2 Tr|1.an2 Tr|1.weight.an2
But if you look in the .asr file, you will see ASREML actually renames
So you must use this new form in the structure specification
Thanks again for the prompt responses.