First of, I don't know Sagepay, so I'm just going on guts and what I am saying might not make any sense (due to you knowing more).
That said:
Is the URI that is sent back a GET? So, purely browser based. Or is there an underlying POST/PUT?

Route::any('success', function() {
    dd(Input::all());
}); // For testing only!

Did look for API docs, but couldn't find any without an account.

Already tried that method too, I guess it just posts back as a callback.

Hmm, that looks right to me. Have you tried hitting the route directly, without the query parameter?

Yes hitting the route direct: /success?something

returns -

array (size=1)
   'something' => string '' (length=0)

So i guess its Sagepay not sending request back because it's local ?

Figured it out I had set a $var with the successful URL that had https and it's not cos it;s local, works now thanks both for your help