Right, you can approach T-Mobile or SAGEM to get it "legitimately" unlocked (not cheap - maybe about £20-£25) or you can go to a private dealer like somebody with a small mobile phone business or a market stall and they'll often do it for about £5 or £10 OR you can get unlock codes from an Internet website - just type 'SAGEM unlock codes' into a search engine - this will probably be even cheaper than a market trader.  E-bay has people advertising unlock codes plus instructions for various phones. 

If you go to a private dealer, ensure you have your Vodafone SIM with you and check it after they've done the unlocking procedure before you leave the market stall or the shop.  I've heard of some phones being unlocked from one network and then they worked with EVERY other network, EXCEPT the intended one that the owner had a SIM for!!

Finally, did you know that there are only 5 mobile phone networks in the UK? They are Vodafone, ORANGE, T-Mobile, O2 & "3".  Every other service provider of mobile telephony uses one of these main 5 networks - they "rent" network provision from them. Tesco Mobile use O2, Asda Mobile use Vodafone, OneTel also use Vodafone and T-Mobile supply coverage to a great many smaller service providers including VirginMobile, Fresh, EasyMobile etc...  A phone that is "locked" to one or other of the major networks will often still allow a SIM to be used from a service provider that's hosted by the network the phone is locked to (this isn't always the case though, so check!)