Today, thanks to this post http://www.hotukdeals.com/item/553854/t-mobile-120-usb-pay-as-you-go-broa
I purchased* T-Mobile dongle for £8.49 when you purchased £10 top up at the same time.
When I registered it, the T-Mobile site was doing an offer, that when you topped up £10, and purchased a 1 day pass for £2, you got 30 days free!!! I should add, you do need a T-Mobile mobile broadband sim number to register, and its not till you log in and go to top up, that you get the offer.
So in total, I got 30 days Mobile Broadband for £2!!! Bargain eh?
* Getting the dongle was a right nightmare. It clearly states on the PC World website the offer of reserve the dongle online, and when you get to store it will be £8.49 for the dongle and £10 credit, total £18.49. It took me two visits to PC World after having to leave and call Customer Services the next day because their till would not process it. My advice, go during 9-5 hours when they can call customer services themselves for you, as they are quite happy to authorise and honour the deal and also bring a screenshot of the offer with you. A bit of hassle, but most of the really good deals on here always are!