Question: I asked my Swedish bank to transfer to my UK bank 50,000 euros through a SEPA payment. Unfortunately, the IBAN and BIC were old and no longer in use and the UK bank returned the money the same day, i.e. the transaction failed. The 50,000 euros was put back into my account after conversion to Swedish kronor (SEK).

However, the amount refunded was signficantly less (about 633 euros) owing to a different exchange rate being used for the conversion EUR=>SEK.

Should the Swedish bank not have used the same rate for the refund? In other words, should not the sending of the wrong codes have simply resulted in a reversal or cancellation of the transaction?

Answer: 
Most likely this will be an expensive lesson to learn. However, I believe there still is a chance to recover some of the difference, since your wording of "*my* Swedish bank" and "*my* UK bank" implies you have a relationship with both banks.

If your Swedish bank converted the funds to Euros first, and then attempted the transfer, then your Swedish bank made a profit when converting it back. If the UK bank received SEK and did the conversion, then the UK bank made a profit by not returning the full amount. Either way, you may ask the bank that profited off of you if they would be willing to make you whole again. If it's the UK bank that needs to do it, I'd guess they would be more willing upon receiving the transfer again properly.

Good luck, and please let us know if you do end up getting some or all of it refunded.
What the bank 'should' have done is a question of law, but I will point out that the reason for the rejection was not the bank's error, it was yours / the payee's [which, in the bank's eyes, is still kind of your error].

Consider from the bank's point of view:

(1) You release 50k SEK from your account;

(2) The bank, seeing it is headed to a EUR account, converts it to EUR for you, at their available rate [of course, they will take a cut off of the conversion];

(3) The bank sends the EUR off to the account you requested;

(4) The EUR is rejected due to invalid account;

(5) The bank needs to convert the EUR *back* to SEK for you [again, they do likely take a cut], at a conversion rate that happens to give you 633 less SEK than what you started with.

If the bank did as you ask, and used the same rate to convert your refund in step 5 as your original conversion in step 2, they might have lost money. ie: if the SEK had strengthened in between step 2 & step 5, then it would have cost the bank more EUR, out of their own pocket, to get you your SEK back. They are not a charity, so they chose to charge you the regular conversion rates in both steps.

Whether they *should* do as you ask is probably a question of law; whether they *might* do as you ask is probably a question of customer relationship. That is, if you were worth 633 SEK to them, they might swallow their pride and give you your original value back, if you asked nicely.