Here's what they don't tell you:
(1) There is an extra PAYMENT REFERENCE option which is ONLY used if it is a SEPA payment. An entry here for a non SEPA payment is ignored.
(2) The costs and instructions box, enter AUTOMATIC and if you go back to that screen from the previous (i.e. step 2 back to step 1 say) remember to reset this pull down list to "automatic" because it will have set itself to something different.
(3) The date for payment MUST be at least tomorrow, otherwise it won't go SEPA. So set for tomorrow and it will actually be sent the next day. Bank holidays not included. So instruct today for tomorrow actually pays day after that; day 3. Bit sneaky really.
(4) Discovered recently that if you are paying say a French dentist (in France) and they give you their bank's SWIFT/BIC Code, you may find SEPA is not offered. I suspect this catches out a lot of people. What you need is the Bank SWIFT/BIC for the branch, usually head office say in Paris, that accepts incoming SEPA (i.e. international) payments. Said dentist will have no idea about this. You MUST of course use the correct IBAN so the right person receives your money.
If successful in setting up the SEPA payment you will get a green box with a white tick in it and the words:
"Your order has been received as a SEPA order. It will be executed on 09.04.2009, provided sufficient credit is available and the order was transmitted within the UBS acceptance deadline. "
(Before that box you will get a similar box saying it will go via SEPA for 30 cents Euro.)
If you do not get this box you may well get a box telling you what you must do in order to make the payment meet the SEPA requirements. Such as "automatically' allocate charges or change the payment date. It won't tell you the SWIFT/BIC code is inappropriate (it may be a perfectly correct code, just not the right one for a SEPA payment). A clue to this may be that when you do the SWIFT/BIC bank search, the bank details look incomplete, so that's a bit like "spot the ball".
Overall, if you know it is a payment capable of going via SEPA, stick to your guns, keep playing until you get it right.
And good luck - I have discovered that even payment from EUR to UK GBP a/c can seem to go via SEPA. Pleasant surprise.
Of course none of this seems to be documented anywhere.
Anthony