At step 1, I was able to twist and push a strong but thin screwdriver (Philips 000) through the plastic parts which would otherwise block access to the screws. By angling the screwdriver slightly the screws can be released without breaking the plastic completely, so you still have the retention of the usb dongle - although obviously this isn't essential.

If you have to heat-shrink connection of the original plug and new battery wires they may be too bulky for the board to go back properly in the carrier. I had to cut away small bits of ribs on the carrier

Thanks for the useful instructions

Stephen