I had the problem of sending large amounts of cash over when purchasing my Villa,after a couple of transactions and both involved paying large amounts of commission,I made a few enquirys,via the bank etc.and came up with the answer that it wasnt the bank charging commission,but the SWIFT system,which charged on any transfer form 50,000eoros upwards,I solved my problem by transferring 49000 euros at a time,result no charges
As for smaller amounts; I use Rational FX to to buy my euros from normally in 5000 pounds at a time the rate is always close to the international bank to bank rate,I transfer the sterling into their bank account on line and my euros are normally in my Spanish account the following day.I get update e.mails from them regulary telling me if anything in the financial world is about to happen that will change the exchange rates in the near future,normally this advise has been mainly been correct