I never buy any seafood that is in packaging because you can't tell if it's good before you buy it. Seafood is so perishable and must always be handled properly in shipping and at the store. In the US, I would buy it this way because when I ask there, they open it up for me and let me smell it. However, in the UK, I've NEVER had anybody (except Waitrose) do this. They think I'm crazy. (I guess just 'ugly American behavior'. )Please don't take this the wrong way, but I can't understand why you folks in the UK don't take up for yourselves better and demad better service.
Also, do you have the receipt still? Better if you do, but in any case I'd ring the store right away (before sell by date expires) and tell them about and ask for a refund, or at least store credit.
I don't hesitate to do this if I have a problem and have never been turned down, but sometimes have to be persistent..esp. with Tesco, which is why I try and never go there. This is completely acceptable behavior in the US, and there you would bet a nice apology and might even get other store coupons or such. Again, you might have to go all the way up to the store mgr...just ask for them in the first place.
At the very least, if you get nowhere, take the fish to where you purchased it to cust. svc. if you can today.
Of course I only do this if a problem and not over due date, and don't make things up. Since it rarely happens, I've not had any problems. Good luck.