I'm wondering if there is a particular type of call or text (to other giffgaff numbers, free or otherwise, or to other numbers beginning with certain digits) which means they bypass the billing system.
Oh, wait a minute. Is this an iPhone? iPhones can and frequently do bypass giffgaff billing by routing texts via iMessages -- which means they go as data not as texts -- and if you're in range of wifi at the time, that means your texts might bypass giffgaff altogether.
The same could happen with calls if you use skype or similar.
In that case, you would have to be careful to disable those features when you make your calls or texts that are specifically intended to keep the service activated.
No I don't have an iPhone @harrrrrry, and yes the charges are on the usage for the last month. 60p for texts, and 43p for international calls and texts. Not exactly a king's ransom, but still indicating Sim card use !
There does seem to be a glitch in the system somewhere !