LilaDuncan@gmail.com declared for all the world to hear...
> Thanks for the advice everyone. I tried a friends O2 PAYG SIM in the
> phone and it reported "No SIM". So, it seems that the phone is locked.

The message "No SIM" would indicate a fault with either that SIM or the
phone itself.

A locked K750i with an incorrect SIM in will say "Insert correct SIM"
and that is exactly to the letter.

> Instead of changing to an O2 SIM can I change the phone from an Orange
> contract to an Orange PAYG SIM?

No, but if you go to an Orange Shop SIM cards are free.
--
Regards
Jon