Heads up for anyone on Vodafone pay-as-you-go with an iPhone, or who's thinking of going for this. This may be a similar issue for anyone with a smartphone (at least, the first point).
a) Vodafone's PAYG mobile internet settings are not the same as their pay-monthly ones (as noted in this post on the Vodafone forums
, the APN is pp.vodafone.co.uk, not wap.vodafone.co.uk as for pay-monthly).
b) If you buy an iPhone from an Apple store it will be set up to use the pay-monthly settings because seriously, who buys an iPhone and then spends less than £5 a month on calls and texts (having run the numbers on the call logs from their old phone), of course you'll have a contract. If your new phone doesn't work despite a Vodafone rep telling you it's been set up to have internet access, this may be the problem.
c) I installed the iOS update over wireless at the weekend, and somehow the mobile internet settings got set back to the monthly ones; if you've suddenly lost internet despite your phone telling you it can find a 3G connection, this could be the issue.
* for pay-monthly the web and MMS APN is wap.vodafone.co.uk and the user/pass is wap/wap
* for PAYG the web and MMS APN is pp.vodafone.co.uk and the user/pass is web/web
The rest of the web/MMS settings are the same.