Why don't you at wiggle specify a list of compatible cranksets and cassettes for each chain you sell? There are many parameters for drive components and it makes it really difficult without this basic information.
It would be a lot more straight-forward if the customer could specify there existing components and filter down to just the set of compatible ones.
This information is often available from manufacturers like, for instance, shimano so there aren't really any excuses. Yes, it would take some effort, but this would be less than: dealing with all the compatibility questions; returns from customers who bought the wrong product through lack of information; not to mention the loss of sales because customers can't be sure that things will work. Also having to ask on a case by case basis and wait a couple of days for a response isn't that helpful.
Someone there should do some forward thinking as it would really improve customer service.