There's a plastic gear in the release mechanism that's prone to breaking. It sounds like that's what's happened to you.
The complete lock unit will need replacing. RRP from a dealer for the part was £50 + VAT when I replaced mine about 3 years ago. It's a 10-15 minute job to replace it so if you have it done at a garage you'll probably pay their minimum labour charge, whatever that is (usually 0.5 hours).