T-mobile is a little more difficult than O2.  You get a swipe card to top up with but you still need to select the usage you wish to use to deduct from your credit (daily, weekly or monthly).  This is easy with the connection manager software that comes with the dongles but all the software does is take you here www.t-zones.co.uk/apps/ebdc/en/purchase-redirect.

So its simple, get your dongle, use the swipe card and then visit the web address above and your good to go.  Your dongle will not be activated until you top up and after top up it could take up to 1 hour.

T-mobile also let you know that you can top up by using “my T-mobile” online.  However, this involves getting a pin sent to you by sms so you will have to use a Windows or Mac machine to receive this or put your sim card in a phone that will accept it.

This is slightly more hassle that the fantastic way O2 PAYG mobile broadband works as you just need to visit the website https://mobilebroadbandaccess.o2.co.uk pay and go.  No content locks or hassle involved.  With T-mobile though, you get a much better fair use policy on internet usage and you do not get charged for going over your usage, instead they may block video sites and downloads between he hours of 4pm and midnight, whereas every other provider will charge you stupid amounts or stop your usage.

Just as a re-cap, the connection deatils for T-mobile PAYG mobile broadband are

APN: general.t-mobile.uk
Number: *99#
Username: user
Password: pass