I have been using a Tesco sim only in my unlocked phone with the facility of payg, I have used up all of my credit &bought a new phone on monthly contract with O2, do I need to notify Tesco that I no longer wish to use payg ?
No there may be a time you want to go back to it. I've always stuck with O2 but it's only a sim card and not a contract with Tesco so no need to inform them.
If you've transferred your number it will already be obvious to Tesco Mobile anyway.
If not, Tesco will just assume that you're not using your phone until, at the end of the relevant period of non-use (which is usually between 3 and 6 months) they'll automatically terminate your PAYG facility. (That would happen after a prolonged period of inactivity anyway, even if you'd still got lots of credit left with them)
Indeed, but it is the network and the gubbins that comes with it that constitutes most of the actual phone service. No company buying wholesale from a licensed provider is going to spend more than they need to, to badge their offering and set their tariffs.