[IainG]

Hi
No harm in keeping the new fuse as well, it is just another layer of protection. If the new unit fails it should just blow its own fuse and leave the main circuit live.
I made up 2 small "Y" harness's with male / female crimps so that the socket still remains live and usable, and if needed the plug in harness can just easily be removed and all goes back to original.

The male ends of the harness's just plug into the socket on the loom, the female end just plugs back into the socket (keeping it live) and the 3rd wires can be inline crimped to the new unit.