I suggest you pop in to your local phone shop and get them to have a look, it sounds like you have no memory card and you have filled up the phones memory.
As the Orange San Francisco is retailed with only 256Mb of internal memory, it has a problem as "Unless your phone has Droid 2.2 or higher the apps etc are installed on the internal memory (which fills quite quick)", this means that although you haven't downloaded "many" apps they're being stored on the internal memory..... Install SD card, then move your app to the card.