Npower, yet again

So after Npower's previous screwup, they've done it again.

To recap, In January 2013 I switched my gas and electricity to Npower.  They set up the direct debit and everything, but sent me no paperwork at all until seven months later when I got my first bill... and it was enormous.  Coz as it turned out they had put me on completely the wrong tariff and hadn't actually bothered to charge the direct debit (I hadn't noticed).  It was merry hell to sort it out and took a considerable amount of my time, with them trying to the deflect blame back to me at pretty much every step (e.g. yes they had forgotten to charge the direct debit, but apparently that was my fault, because I'm apparently responsible for checking up on them all the time to ensure they've done their job.  I do wonder what the advantage of direct debit is supposed to be over just paying manually every month if they think I have to check it every month anyway).

Anyway, it eventually got sorted out and I paid off the hundreds of pounds of debt that had amassed on my account.

In January this year, I got a letter from them saying I was overpaying and that they were reducing my DD payments by about half.  All well and good.  However, I now have a letter telling me I am hundreds of pounds in debt to them (again) and that I must pay this within 9 days of the letter date (actually, I must pay it by 26th May, but of course I didn't get the letter on time because I was away from home).  It turns out that rather than reducing the DD payments, they just plain stopped them altogether (again).

So clearly its their fault that I've ended up with a load of debt against my account again (although I'm sure they will insist that its my fault), but even if it wasn't their fault, how is it right that they first thing they've done to let me know is give me 9 days notice to pay up?  Surely if the account is set to be paid by DD but isn't actually being paid by DD then they should tell the customer immediately rather than waiting until the account is a few hundred quid in debt?

Needless to say, I've now switched energy supplier and am waiting for Npower to call me back to reach a resolution for this...  Never before have I dealt with such an utterly incompetent company.

(Edit: Having just got off the phone with them, they have applied the £88 direct debit discount that they "forgot" to deduct from my account (despite me querying it in January already) and have let me pay off the balance so I can switch supplier.  I await the revised bill that they are sending through so I can check it's actually accurate this time.  I do wonder how much money they make off "forgetting" things like DD discounts and hoping that people don't notice.  Maybe I'm just cynical, but I feel like there isn't much incentive for them to fix their systems when the billing errors are making them money.)