you might be surprised that most high street retailers do not pay commission to their staff anymore. As mentioned the 6 year rule only covers faults that can be proven to have been there at the time of purchase and the onus is on you to prove it, at your own expense. Also manufacturers guarantees very rarely cover natural wear and tear, whereas some companies will offer extra cover that will cover wear and tear beyond the expected life of the product.
Also compare what the 'extra' bit of the cover is. Is it extra time or extra cover. Did you know that manufacturers guarantees on computers don't cover the software so if the retailer does then it might be worthwhile. Some extra cover is a rip off, some is not. When they try to sell it to you listen, judge whether it will benefit you and use it as a bargaining tool for dicount. Especially now some retailers are using a month by month option. I got forty pounds off one item by agreeing to an �8.99 a month extra cover which i cancelled before the second payment so i saved �31.