It turns out, I had done something really stupid.
My SSIS package, as it turns out was perfect, I had named the transactions and set "RetainSameConnection" to true. Begin/commit/rollback commands were all specified correctly.
My problem was that one of the stored procedures called as part of the SSIS package had a unnamed transaction in it, causing SQL Server to commit everything once the commit in that stored procedure was called.
Resolution was to modify the stored procedure to name the transaction, so that SQL Server treats that transaction as a nested transaction (In theory, I would have thought the second "begin transaction" statement would do this, but for some reason this was not happening).
