usb cable charges iphone but PC does not recognise iphone to offload photos

Does iTunes recognize the phone? If not, refer to this: If iTunes doesn't recognize your iPhone, iPad, or iPod - Apple Support

If iTunes recognizes your iPhone follow the instructions here: Copying personal photos and videos from iPhone, iPad, or iPod touch to your computer

... and if you don't have iTunes installed on the PC, you'll need to do that first so that the "Apple Mobile Device Support" software (comes with iTunes) is installed to allow the PC to recognize the iPhone.