Hey Dav...i am a serial nail biter. Have always the past 28 years. I don't know what I can say to help you stop. I have tried to stop only to start again. It's comforting and just can't stand long nails.
Right now I've stopped biting my nails for 3 weeks. It's a long time as you know. My nails are very long for me. I just decided I wanted to stop biting my nails. Short nails look bad and unprofessional. I can pick up things off a table cause I have nails. I used nail hardener to make them strong but that alone won't help you stop. You just have to decide to stop biting them.
Just like some people need alot of things to try and stop smoking and others just do it cold turkey. Guess I decided I needed to stop and have done it so far. But it's very very very tempting to bite them again.