I've never been charged any fee I didn't get back, partly because I'm careful and mainly because I don't think being overdrawn for 4 hours warrants a £30 overdraft fee. A stern letter is usually enough.
I wouldn't have signed the mandate, not because I'm a greedy feck (and eyeballing a nice axminster planer thicknesser) but because while it wasn't your money, it wasn't your mistake either... let me explain...
2 years ago the builder guy I live with had 50k - yes £50,000 drop into his account overnight. Needless to say he was surprised to say the least, he ummed and aahed for 2 weeks before finally bringing it to Natwests attention, who a day or two later traced where it SHOULD have been sent and redirected it. (same name, 1 digit wrong; and the proper recipient didn't offer a reward either, cheapskate.)
I told him the same thing, take the money and sit on it in cash - the ONLY way banks will learn to take a bit more care with OUR FFING MONEY is if when they do screw up - make it sting - make them foot the reimbursement costs, tell them they should consider it a "business experience fee". If you don't charge enough for a job but the final fee is written into the contract - a lot of the time you have to swallow it, and accept you got your figures wrong - lesson learned. Happens in contract law all the time.
The fact is when the proper recipient didn't get it they would have had serious egg on their face and either go begging to my mate for it back (and I'd have told them to take a running jump) or keep quiet and send the £50k where it was supposed to go out of their own coffers.
Because banks have access to our money, they think they can either just take it OR threaten dirty tactics to coerce you. Frankly RogerS I'd have called their bluff and told them to drop it or I'll go public. In the last few years banks are reeling from one scandal after another, you wouldn't have had to fight hard in all honesty.
If it were I losing the £50k, I'd not care you sent it to the wrong person, I'd not care if the person you DID send it to; kept it; all I'd be interested in was getting my money back, and I'd scream until I got it - and incidentally if they did manage to get it off the wrong person I'd insist they give him a "reward fee" for the trouble they caused him or I'd go public.
but that's just me.