Depends on the store TBH. My local store (Bexleyheath) is useless. Asked them about altering my tariff and they completely screwed it up by not looking @ the info properly (used to be £40pm, after was £180 ). Quick argument later, tariff was reversed and full credit given, plus LOTS of discounts for rest of contract (never paid more than £15 for next 14 months).

I use the Bluewater store and they have always been okay so far. My N95 8GB is in for repair due to internal and external speakers going crackly. IT got sent off, was meant to come back on Tuesday just gone. Went down there and it had not been received back into store, but had been collected by the couriers. Spoke to the manager, he looked into it quickly and said that if it is not back by Monday (first working day after Christmas for courier company), I am to go into the store after work and he will replace the phone FOC with a new one, plus 1 months free line rental for each week I have been without the phone, and put notes on the system to the same effect.

One thing with most phone shops, MAKE SURE they put anything they tell you onto the system. It can be your only proof about circumstances should something go wrong.