Nick wrote:

My father always said that a DD is akin to letting someone into your
pocket. I didn't understand at the time but have always followed his
advice. Pay what you have to pay when you have to pay.
There is no sense in giving these people free loans.
Just my 2d worth.
Nick.

DDs are extremely convenient and with the DD guarantee there is not
much reason to be wary of them. They save everyone a lot of time
and effort. The alternative is to write cheques and post them, or
go to a bank or post office to pay them (or cash) in, or to use
standing orders (which have their own problems[*]); there's also
direct transfer by home banking, but frankly I find that more of
a faff than writing a cheque. DD really is best.

As to free loans, that works both ways (the trick is to join a new
supplier just at the start of the cold season, then they will always
be giving you free loans). Some people find it easier to budget when
they pay a fixed amount each month or quarter, and to have the amount
reviewed every year or two, than having to face big winter bills and
small summer ones.
[*] The problem with SOs is that while they're fine for annually
reviewed budgets, they are not suitable for variable amounts.
DDs are suitable for both.