I know Sainsbury's use the vodaphone network but are their phones locked or could I use my o2 sim in them?

Probably locked...but as you have an O2 sim, why don't you try it?

cos i don't want to buy one if it isn't going to work

Get a free sim e.g. Pay as you go to test it.

I know Asda and Tesco lock their mobiles so I'm guessing Sainsbury's might be the same

I am guessing he means he doesn't want to buy the phone if its a non starter

Probably locked to Sainsbury's in addition to Vodafone (as per Virgin).

Easy and cheap enough to unlock depending on the phone.

So is the Sainsbury's phone under a contract or PAYG? Why would you want to insert an O2 SIM into a Sainsbury's phone? Surely if you are happy being locked into O2 you could get a subsidised phone from O2?

I'm puzzled.

My Galaxy S4 purchased 8 months ago from CPW is away for reoair and CW now tell me it could be up to another 28 days (it's been gone 2 weeks already). The "phone" they've lent me was last used by Fred Flintstone. It will make calls and (just) send text messages (as long as you have about 20 min spare to tap out the message).
My contract is with O2 unfortunatley and was deliberatly bought cos it has a high data amount. I use my phone all the time for work - it has my whole working life on it - and yes it's backed up bu this borrowed phone was made before emails had been invented I think.
Consequently I'm very very reluctantly looking a buying a phone until mine comes back. Sainsbury's have the Galaxy Ace and the Galaxy someting or other pretty cheap but I need to know my SIM will work in them. Oh and by the way, yes all Sainsbury's phones are unlocked.
As soon as this contract is up I'm going back to an iPhone. I actually prefer the Samsung but at least with an iPhone I could have just nipped into town and with a 8 month old phone they'd have swapped it for a new one straight away