I have written in the past about cybercrime including pfishing.
I am indebted to Computing Which magazine for information about a worrying new development.
The latest type of scammers do not send out blanket e-mails addressed impersonally appearing to come from a well-known organisation, but one you do not deal with. I do not bank with Barclays and do not have an Apple Computer for instance.
The new boys study you by secretly reading your e-mails and send you an e-mail addressed to you by name, appearing to come from a firm you actually do business with. They send you an invoice very similar to a genuine one but with changed bank and contact details asking for a payment, when they know you were going to pay this firm for goods or services they had supplied. They may also write to the creditor, appearing to be you, asking for more time to pay, thus delaying any reminder you might get.
The best defence is vigilance and contacting your real creditor using contact details you already have, to ensure the invoice is correct.