Been researching mobile broadband for some time.
Mobile broadband is a mixed bag with some people very happy and others complaining of terrible speeds. Overall, the fastest speed supplier (atleast in London and airports) was Vodafone, but the downside was always their price.
They are now doing a special offer that can only be ordered on the phone. 18 month contract for £18 p/m for 3gb data. Also you get the dongle free.
One downside/upside is that if you overgo your 3 gb they immediately charge you £15 for the next gb - even if you use one megabyte. This is a little sharp, but on the other hand if you were to overgo 1gb with 3, that would cost you approx £100. They told me that their software will warn me when I get to the 3gb limit, so you should only overgo if you want to.
On their website you can get an 18 month contact with 3 months free, but this deal is far better. When I called sales, they first tried to flog me that deal, so make sure to ask for the £10 deal. Even if you were to take the web deal + £40 quidco, you would (if the quidco went through) still be saving £5 with this deal.
You can cancel the whole thing within 7 days if unhappy with anything.
This is not for everyone, but as someone who has been searching some time this is a spanking deal.
Not sure on expiry, but this is a current offer, not permanent.
Review can be found here: http://www.mobile-phones-uk.org.uk/vodafone-broadband.htm
Hope this helps someone.