If the available balance on your current account differs to your actual balance then it could mean that you have a pending transaction on your account. A pending transaction is a payment which has been authorised and removed from your available funds but hasn't yet been charged to your account. This could be caused by a Visa payment or cheque which is still being processed.
If it's a Visa payment, the transaction has been authorised and removed from your available funds but hasn't yet been charged to your account. Until the payment has been charged, this amount will not show on your statement – this will usually happen within 7 days of the transaction.
All cheques paid in via a Nationwide Branch or Nationwide Fast ATM before 7pm (Monday to Friday) will clear by 11.59pm the next working day. For withdrawal and "certainty", if a cheque is paid in to the account on a Saturday, Sunday, Bank Holiday or at a Nationwide cash machine then the day of deposit will be the next working day.
So for example, if your available balance is £100, and your actual balance is £110, then you currently have a pending payment of £10 waiting to be charged to your account. This could be made up of any number of Visa transactions made within the last 7 days that are not on your statement, or any cheques paid in recently.