I am as cynical as EuanC to be honest!
My DH had a rather smaller amount paid into his account a month ago, which he was expecting and the bank was on the phone within days! They werent asking where it came from, although they knew, but to advise he visited his nearest branch to discuss how best to save the money.
I am struggling to believe that in these times where depositing �3000 in cash in account sets alarm bells off that a bank is ignoring nearly �100k a month into a bank account!