Some more information about my seedlings:

I'm in Bristol. I have seedlings derived from
the trees at Bristol University. The trees
in question are in the botanical gardens and
the Royal Fort Gardens. (Are there other
mulberries in Bristol - apart from Pat's and
mine?)

There are photographs of these trees; their
leaves; their berries; and my own seedlings at:

http://timtyler.org/fruit/mulberry/

The trees (and also their corresponding seedlings)
seem rather different from one another.

I /suspect/ the botanical gardens tree is a
Morus rubra - and the Royal Fort Gardens
tree is a Morus nigra - but I'm open to
more knowledgable suggestions on this front.

I also have a single specimin of Morus alba
(which I believe is silkworm fodder).