I had the problem recently. I contacted Morgan and was quoted £170 for the whole assembly, part no 3FL501. I couldn't buy the insert alone from them. I then found my spare unused key was fine, it held locked inside the insert. I contacted Newton equipment, who make the insert and found they sell pairs of spare keys marked 044. My originals are marked 044, which it appears they all are. I ordered a pair and they work fine, so now I have 3 good ones...
https://shop.newton-equipment.com/products/spare-keys-for-zero-range-aa-ac-8?variant=40649837150396
