So my credit card company charged me for two transactions I do not recognise (to skype.com) and I challenged it. They posted me forms to fill in, which I duly did and, being a planet-saving-kinda-guy I re-used their envelope to return the forms (clearly crossing out all their own info on the front of the envelope and adding a stamp).
A week later my card gets declined in Sainsburys (yeah tell me about it, trying to go upmarket like that was my downfall!). Turns out they received the letter, but because it was in one of their envelopes it looked like a returned letter, and standard practise seems to be to put an account on hold if they get any bounced mail (without trying to contact me via another route at all, of-course).
So now they’ve re-enabled it and are sending out fresh forms so I can challenge the transactions again.
This time I’ll find a clean envelope.