When is a 5% discount not a 5% discount?

I ordered a new watch as mine seems to be having issues. It is many years old now. I googled for a new one, saw a range of on-line shops selling them, and ordered from one. I picked Watches2U as they had a good price and the web site looked sensible.

I selected the watch and then got a pop-up offering (as you can see) "enter your e-mail below to get 5% off your order".

There was no caveat, or *, or T&Cs, or whatever, just that I had to enter my e-mail. So I did, and got a discount code by email.

I entered the code and ordered, but the order did not show the 5% discount. Well, actually it did, but showed 5% discount and -£0.-0 as the amount! I queried it and I was told that as per the terms on the email I could not use the discount code on "sale" items (which appears to be pretty much all of their web site).

Of course, I am somewhat annoyed at being duped. Indeed, it seems I have been added to a mailing list as well - without asking me. As far as I can see, they made an offer and I accepted. Indeed, when offering a discount off "your order" I had already selected a watch (that was on sale), so it seems very reasonable to expect the discount to apply to my order. I think adding terms in the email sent after I have accepted their offer is not really on.

The really stupid thing about it is that I would have not had any problem if there was not a 5% discount on offer. It is not a lot of money. But now they have offered me a discount which I have accepted, and then been denied to me, I am cross and annoyed. I have raised a complaint via google now as they are a "google trusted store".

Their 5% offer has made an unhappy customer, even if this is resolved and I get my discount.

Update: Google not interested in fixing, and Watches2U not interested, so reported to trading standards.