Helen,
sorry, it seems to have been my misunderstanding of FK relationships; i.e.

add constraint fk1_prodsupp foreign key (supplierid) references supplier;

is NOT the same as

add constraint fk1_prodsupp foreign key (supplierid) references supplier
(id);

It is this not specifying the field that was causing the problem - as soon
as you add the column specifier everythings OK.

Thanks for your help,
Stuart