Detect specyfic router from url

having this route in config

    routes.MapRoute(
        "auctioneersByLetter",
        "{culture}/auctioneers/{LetterFilter}",
        new { controller = "Client", action = "Index" },
        new { LetterFilter = @"^([a-zA-Z]{1}|all|0-9)$" }
    );

and then in my private method having only current url like domain/en-gb/auctioneers/b or domain/en-gb/auctioneers/b/search-filter?Country=United%20Kingdom how to get LetterFilter parameter from it ?

Tried with UrlHelper but no result