>>> "I have now uses 47GB of my monthly allowance of 1.24 GB"
As it stands, that doesn't make sense (as what you've used appears to be vastly in excess of your monthly allowance!). So I assume that the '47GB' should read '47 MB'.
There are 1000 MB in 1 GB. (Pedantically, it's actually 1024, rather than 1000, due to the way computers work, but that can be ignored here.
So you've got 1240 MB (or 1250 MB, depending upon whether it's the '1.24' or '1.25' figure in your post that's the correct one) available to you each month. If you've only used 47 MB so far, you've only used 4.6% of your monthly allowance up until now, so you shouldn't need any additional top-up at all.
HOWEVER, if you REALLY mean that you've used 47 GB then, as you've only paid for 1.24 (or 1.25) GB, you'd need a MASSIVE top-up to bring you back on track. I simply can't see that's possible though unless, for example, you've downloaded loads of full-length movies in high definition today.