Thanks for your answer.
Another suggestion from someone was
Just do a;
SELECT ProductID FROM WishListTable WHERE ProductID = <selected Product ID>
If you don't get an empty result set back you know the product is already there. Or if you are using SQL Server Stored Procedures;
IF NOT EXISTS (SELECT 1 FROM WishListTable WHERE ProductID = <selected Product ID>)
INSERT INTO WishListTable
...
These set me looking for tutorials and I found one at
http://aspalliance.com/687
I've incorporated these suggestions and the tutorial into my system and it works a treat
Thanks again