The link given in my post above seems not to be working currently, due to work on the Vadafone website.
The link to the pay & go flat rate charge of 50p for up to 25 MB is
here.
Another link which gives both contract and PAYG optiins is show
here.
Interestingly, this conflicts with the price plan for the new Vodafone
Simply PAYG tariff which says it is £1 for just up to 15 MB in a day !
If anyone has used the latter, please post what you have found the charge to be for mobile browsing on your phone.