Ferd:I even called my online bank and they only took a few minuets to answer the phone (nice one) but they could not help
Regarding your discussion with your online bank there must have been a misunderstanding. They must accept your instructions to have a payment arrangement comprising an account number, sort code and a customer reference of up to at least 16 characters. How to achieve that is your bank's problem, not Edf's.
From previous experience I know that a new payment arrangement sometimes cannot be created online with the same sort code and account number as an existing payment arrangement and sometimes the customer reference on an existing payment arrangement cannot be changed online. Also sometimes the template for a particular supplier is not up to date. Possibly the problem in this case. These are bank system's limitations (defects) for which there is an obvious workaround.
You have received very poor service from the particular bank person you spoke with. Go back to the bank with the supplier's sort code, account number and your new 12 digit Edf customer reference and insist on a payment arrangement being setup manually, if that is what you want to achieve.
Regarding Edf, my advice would have been to wait until you were informed of a change. If the supplier screws-up your payments to the old number without informing you, kick ass via their complaint's procedure.