Ah, so my port number is... something completely different from what I thought it was, d'oh! Having done what you suggest I now get a different error on the BML page, 'Internal Failure We're sorry, but something failed internally on our end attempting to process your payment. Your credit card has probably not been charged.' And attempting to run a gearman worker script by hand gives this: 'First gearmand server in @LJ::GEARMAN_SERVERS (127.0.0.1:17096) isn't responding.'

So... this takes me one more rung up the ladder of debugging, I think..