jonnoboy,
Worry not, tom24 is absolutely correct. After 5 years your conviction becomes 'spent'. The rehabilitations act of 1974 clearly states that of you are asked the question "have you ever been convicted of a criminal offence", you are allowed to say "no sir, I have not" if your conviction is spent. This is true whether it be on an apllication form or during interview.
You have nothing to fear, airlines will never find out unless you really want them to. Even then, I guarantee you it will not rule you out...
So as tom said, get on with your training and good luck.
J