There is a factory made filling loop for the Baxi boiler however most of us engineer's do not bother with them as they are very expensive compared to the standard filling loop we normally fit into the pipe work. So if you have a factory unit it will be fittes directly underneath the boiler to the left hand side 22mm return valve and a small bore 8mm copper pipe will loop to the second from left 15mm cold inlet valve with a double check valve and a isolation valve in between them. you will require a flat blade screwdriver to open the isolation valve and increase the pressure.
As i have said though this may not have been fitted and therefore you will have to look along the pipework and somewhere close to the boiler you should find the filling loop it will look somthing like this.
http://www.screwfix.com/app/sfd/cat/lrgimg_pop up.jsp;jsessionid=PD2HW042XZNOOCSTHZOCFEY?prod uctId=13806&imageNo=null&ts=20573