Im wish to leave tmobile next month and switch to tesco mobile. My tmobile contact sim will goto a pay as you go sim from next month so am I best asking tmobile for a PAC code before it changes to a pay as you go sim or after or does it not matter?

It does not matter. The PAC has only a limited life - 30 day's I think - so don't ask for it until you need it.

It should not take more then 2 hours to get PAC code from your current supplier. I think when my dad switched to tesco a couple of years ago it took 24 hours for the number to change over.

http://ask.ofcom.org.uk/help/telephone/PAC