I recently had cause to complain to Tesco, well it was actually about a third party delivery company but didn't get any joy from them so went to Tesco as it was them that I had ordered the item from.
I have to say that their complaints or customer service team are absolutely first class and it has actually made me shop with them more than I did before.
As far as complaining about the slice - I'd bin it and chalk it up to experience, wouldn't bother going back unless it happened again.