Direct Debit Guarantee on new DD?

Hi all.

A little background;
Some time ago I entered into joint loan at welcome finance with my partner, last year we split up, which then left the problem of who pays the loan (Welcome weren't able to split it in any way)
My ex decided she would but after a while their were difficulties (she was a student nurse, and had problems with wtc)

So I took on the loan.

Now, I ought to explain that my Debit card account is with Bank of Scotland, but my 'main' account (where all incoming moneys are paid to, and DD's go from) is with Nationwide, an account which doesn't have cheques or debit cards.
This is ideal for me as I have had problems in the past, it allows me to have a system.

Anyway, I started paying using my debit card from Bank of Scotland, and went about setting up a Direct Debit with Nationwide.

Last week I paid the regular payment to Welcome with my debit card, as I had not been informed of when my DD would be set up (this has now been 3 months).
Yesterday they attempted to take the DD from my nationwide account, but this was refused due to insufficient funds.

I have a few questions;

1) Under the Direct Debit Guarantee shouldn't welcome have contacted me informing me that the DD was active?

2) Shouldn't welcome have informed me of this when I was making the debit card payment?

3) Will the DD be re-presented?

4) My bank will charge me for the DD being refused, would it be best to have that cancelled or to use the DDG, and if the latter, how do I go about it?

5) I have also read that companies such as Welcome have been known to 're-use' Debit cards; but each time I paid, they would ask for all my details, so this wasn't set-up as a recurring payment, but for future refrerence, can it be?

Thanks.