It may be worth pointing out that these tamper proof tax disc holders can only be used once (i.e. every time you renew your tax you'll need a new holder). So I'd order a few... Bare in mind though that most drivers will only need one a year.
If you've not seen them before you place the supplied sticky back plastic circle over the front of the tax disc (making it 'tamper proof'), stick that to the big rectangle, which in turn you stick to the windscreen of your vehicle. I'm not sure how suitable they are for motorbikes with out windshields, I suppose you could just use the clear plastic disc and your standard disc holder (then at least it is tamper proof should anyone steal your tax disc).
Note that this is a Department of Transport scheme, so technically you've already paid for them (through road and fuel tax etc) regardless of how many you order.