I have lost my master key, what do I do?

I have lost my master key for my T reg Vauxhall Tigra 1.6, I have a spare key (have been told that it does not have a chip in it) which locks and unlocks the car but cannot start the engine because of this chip it is missing and have been told that it scrambles the immobiliser. Do I need a new ignition key and use the spare to open the doors, can I disconnect the immobiliser, or should I get a brand new set although money is a big factor.