In addition to the great info above, here's some boring info that might save you amassing bank charges. In theory, every single transaction that you make with a Visa Electron card has to be "authorised" (approved) by the bank. If your bank balance is �8.55 and you try to spend �9 it could be declined.
With the new and 'improved' Visa Delta card, this isn't always the case. Different shops have different "floor limits" and if your payment is below their floor limit (say theirs is �20) then your payment is approved by the shop alone/locally : the bank possibly don't even know you've made the payment until a few days later.
This way, you could spend that �9 and go a whopping 45p overdrawn and be charged about �50 for the pleasure.
A bargain at twice the price!!
The 2 morals of the story are :
1. Banks are a bunch of thieving gypos.
2. You need to make sure you have enough money in your account before spending. Don't assume that just because your card wasn't declined that your balance is fine.
Finally, Visa Electron cards never (to my knowledge) double as cheque guarantee cards whereas a Visa Delta card often does.
Prizes for anyone still awake!