All I did is added the searches to the side bar/reading list. I wouldn’t get alerts but I just clicked each website, each day and my target cars would come up.
You could try widening your search (e.g. keyword = walkinshaw) and setting up alerts for that until you get one that contains the specific you are looking for (e.g. W427) and then set up that alert. Here's a starter CarSales search for Holdens with Walkinshaw in their details. (44 results, none contain "W427")