what's affecting this line strength? temperature? distance?
Length.
i could reinstate a master socket at the original location and place the router there. i expect thats probably the best idea, but i'd really like to have the router at the end of the extension.
I'm sure you'd also like the speed back. As I said, put the router at the master socket and run cat5e to the new location.
because this extension cable is outside for about 25m does that mean its useless?
No, but it's likely longer than it needs to be, and will not likely last very long installed externally.