It takes BZWBK about 60 seconds to generate the csv file for our account, I know this because I recently had to increase the delay in our script.
That being said BZWBK has far better systems than any of the UK banks.
Aren't you using Alerty24 for BZWBK incoming transactions?
These are E-mail's sent instantly when a deposit comes to your account. These are OpenSSL signed, so you can actually verify the digital signature - ensuring that the E-mail really comes from the bank. That way you could credit deposits instantly without logging in to the website.
That would be absolutely awesome, it means that customers having accounts in BZWBK (which a lot of people in Poland do) would be able to deposit to Intersango instantly, which would surely increase attractiveness and thus volume. I noticed you recently lost a lot of PLN volume to MtGox, but instant crediting would bring a lot of it back.
The E-mail has all the data you need - including REFERENCE field.
It has a S/MIME Cryptographic Signature with the same certificate as the website, so its as safe as logging in on the website, but maybe for larger deposits you could log in after receiving the E-mail anyway to double-check.
Think about it, it would be definitely awesome.
And anybody in Poland who still doesn't have one, can walk up to a BZWBK branch and open a free BZWBK account in a few minutes, and instantly deposit cash to his account and then make a transfer to yours. It means anybody in Poland could buy Bitcoins instantly.