You have to "formally" request it to end by giving 30 days notice or O2 and ALL networks will charge you for that extra month
Eg contract anniversary 28th of month it will NOT end on 28/03 [ less than 30 ] but 28/04/2012 if you called today.
Ring up O2 and request a PAC, these last 30 days from it being issued [so using the above you need to request the PAC near 28/03/2012 or use it by 05/04/2012 30 days!]
(it is best to advise Tesco Mobile of (PAC) when the O2 credit is very low as any unused O2 will be lost)
The Tesco Mobile sim will have a (new) number eg 0789Tescomo and so your (present) O2 number after porting is on TM and 0789Tescomo will cease also
Both numbers are 'live' so both are capable of receiving/ making calls until the port
At the time of transfer the O2 sim will "die"**** and the Tesco Mobile comes alive (not in the Walking Dead sense but you know telephonically) with your number.
****As PAC is designed for 2 phones you do NEED to monitor the O2 signal as when you have NO signal you have ported and need to then use the TESCO Mobile sim card.
Here is something I prepared earlier
http://ask.ofcom.org.uk/help/telephone/PAC
IF THIS IS THE 02 HANDSET IT WILL NEED TO BE UNLOCKED