Hi Pauline,
I suspect you will find reference to this in your terms and conditions for the account.
When you set up a Standing Order, this is an automated payment from your account on a date specified by you.
The LTSB wesite says the following...
If you try to make a payment but do not have enough available money in your account, then we will either agree to an Unplanned Overdraft covering that payment or you will not be able to make that payment. Fees will normally be charged in either case. Please see condition 10 in ‘Your Banking Relationship with us’, the Personal Banking Terms and Conditions, for more details about overdrafts and available funds.
It is still your responsibility to ensure that you have available funds to cover any payment that you have requested. I am surprised that this was not made clear to you when they waived the fee the first time this happenned.
Hope this helps...