The issue number is a single figure, usually found at the bottom of the card, to the right of the expiry date.
Sometimes, even though there is a field for you to input the issue number when you are buying on line, it isn't compulsory to put it in. If you can't find one on your card, try skipping the 'issue number' field and you will probably find it will accept your payment.
Hope this helps.
Worked for Lloyds TSB for twelve years