Both escs flashed!
In case anyone else attempts the same I used the kkMulticopter flash tool which was incredibly easy to use, and the key to getting it working was using Zadig (http://zadig.akeo.ie/) to install the correct driver for the USBasp, and when installing the driver I had to select "libusb-win32 (v1.2.6.0)" as the driver. initially I chose the wrong driver and it would not work.
Another top tip that I only spotted after doing the first esc, there is no need to remove the heatsink, just leave it stuck on.