This, however, should hopefully be the last instalment, as, when I opened my post today to find some more credit card cheques from them, I just phoned up and cancelled the card. It seemed like it was the only sure way of stopping any more arriving (although I suppose I'll have to wait a month or two to see if that's true!)
In beautifully ironic style, they were fast to answer the phone, and courteous and efficient in closing my account; it only took about three minutes. Ho hum.
I doubt I'll manage to get another credit card with as high a credit limit on it in a hurry, but I'd never pushed it to more than a couple of grand over the last few years, anyway. Maybe this is an opportunity for me to rein my shopping habits in a bit :)
Oooh, and the comment I just found here suggests I wasn't alone in the problem, at least: "Have been in contact with Lloyds TSB for quite a few months trying to stop them sending me credit card cheques through the post. Numerous phone calls and promises to sort situation out - finally wrote to head office - definitely no more cheques through the post. Two weeks later another 4 cheques arrived they appear to be sending them out at the beginning of each month. Am now in touch with Financial Ombudsman, awaiting outcome. I have copies of all correspondence."