The account I have from my mifi sim appears to have someone else's address attached to it, in the past when I've topped up a different address has appeared, again not mine, but as I went through the top up process and it worked fine I though no more of it, apart from the fact that vodafone was in breech of all sorts of data protection laws.
Yesterday I tried to top up with a credit card as usual and it failed saying the details don't match, it' let's you put your address of the credit card in so it shouldn't matter that the account address was different, tried again still no joy.
So attempting to get the address changed, which it appears you can't do yourself, I spoke via the chat service to Dave who said the change of address should take about an hour to go through. So as I checked the account this morning, no change to the correct address, I'm not going to even attempt a top up until the account address is correct, even though it may be unrelated to the fact that I can't top up, but it'd just too coincidental.