Re-reading my rather vitriolic rant on Saturday, it began to dawn on me that perhaps I'd been more than just a little unfair. Irrespective of the rights and wrongs involved, companies aren't psychic, and often the easiest way of getting changes made is simply to ask for them. With that in mind, I popped into the Coventry Building Society local branch this morning, spoke to a very pleasant and sympathetic guy who said he'd "see what he could do" and within a matter of minutes my £40 was refunded - albeit it with the caveat that "we won't be able to do this again". Fair enough.
So I'm once again a happy customer. Props to the Coventry for living up to their slogan, and for realizing - perhaps - that a happy customer is also a loyal member.