my legal perspective remains as it was when I signed up.

If someone signs up via the cashback site (even with your post in mind, that is all thats needed) and then gets told their transaction isnt legitimate then that is a broken sales contract which in turn makes the service contract invalid as well. So if plusnet were e.g. exercising their right as you call it to deny a payment, then the customer can likewise cancel without any penalty whatsoever and have a full refund of 'all' costs.

Only if the law agrees that you hadn't already broken that part of the contract by finding the "product" by another means, such as a search engine. Quite how this would work in court I have no idea and quite possible the judge would come down on the customers side but they may not.

It is the continuing reason why I don't use them as I would always be breaking that term.

where does it state in the sales contract that one must have discovered the product on the cashback site?

Conversely, where does it state in the sales contract that the cashback from Quidco or TopCashBack is anything whatsoever to do with the contract between the customer and Plusnet?

In reply to a post by Chrysalis:

where does it state in the sales contract that one must have discovered the product on the cashback site?

I'm sure it doesn't. As I pointed out earlier, Quidco (and maybe other cashback sites) even has a toolbar to alert you when you are on a retailer's site that cashback is available if you go through Quidco.
http://www.quidco.com/cashback-reminder/

TCB has this:-

http://www.topcashback.co.uk/Help/178

I'm not sure I like either of those plugins. The user is setting up yet one or two more sites that can track every web-site visited..

Not for me, thank you .

In reply to a post by Chrysalis:

where does it state in the sales contract that one must have discovered the product on the cashback site?

I already posted the details I found on TCBs website. Are you saying that they are wrong? They seem to be quite clear that cashback may not be paid and that the supplier can refuse it if they believe you used a different route to find them.