What was that form where I entered my bank account number?

That was helloBill's own form. You used it to choose what to set up or switch (energy, water, council tax, broadband or your TV Licence) and to give each provider the details it needs, including the bank details for your Direct Debit. It's part of the secure helloBill journey, not a separate company. See "Are my bank details safe, and who holds them?" for what happens to them.