yes, a signature from the account holder )or holders if joint account) is required to authorise and initiate payments. As is full bank details (ie acc no. and sort code).
no, a signature is not required in every case. This is how come it is possible to set them up online (eg for phone bills etc), as I have done in the past
5029...I believe that what you have set up on-line is a standing order. For a Direct Debit you will need a signature. Don't ask me the difference between the two though !!
I think (tho am by no means certain!) that a DD can be a variable amount, and the people you pay will request it from your bank at a regular date each month. A Standing Order cannot be a variable amount, it is the same each month and your bank will deposit it with whoever you have instructed them to pay. You must set it up for a specific period, whereas a DD can be taken from your account until you cancel it (or the people you are paying stop requesting it).
For information, a DD is initiated by the firm collecting the money, and is basically authorisation for them to take money from your account, and the amount is variable ie up to them. A SO is set up by the account holder and is an instruction to his bank to make a regular payment to a specific beneficiary.