I'm not a solicitor, but a legal secretary, but really, not answering from a legal perspective, but I don't know that you would need to put a conviction so old on your visa application. I just was recently looking over the naturalisation forms I need to fill out and they ask if you've had any convictions (including road violations) in the last 5 years. I would double check the application form and look at the wording carefully. If it doesn't give a timescale then I would go ahead and list it and I would simply put in the information you do have and remember. If they feel the need to question it, they will contact you but I'm guessing that as it is so old, that they won't really care about it so much, especially given the sentence was merely 6 months probation.