Customer service answer....

Well apparently the reason for credits this month are because last month's bill was wrong - probably true but as others have said you can't actually check the itemisation online to confirm :-( Next months bill could be interesting

Understatement!

I don't need to look very hard for a problem with my bill! They are being kind enough to invoice me for a grand total of £1.82 this month.... which includes a £17.65 *credit* for a "3G GPRS bundle" that I have never had plus a credit for a 2nd month of "Mob Internet Free Trial" which they gave me last month. The previous invoice had a credit of £2.02 for mobile data in the States that I hadn't been charged for to start with.

It wasn't worth the hassle to query last month but I think I will see what explanation they can dream up for this month's effort :-)