I've just joined the site specifically to answer this question. I am on the Asperger's spectrum and it's a non event. There are no boxes to tick for it. If you were refused a license, it is because you'd need to send a separate letter to basically say 'you didn't ask for it on the forms, but I'd like to point out things you don't know about me, to see if it's an issue, it isn't, but I'm going out of my way to persuade you it is.' It would be no issue, even if you volunteered this information that no one is asking for. You'd have to work hard and be very persuasive to MAKE it an issue?
I have over 300 trials trophies and competed, many moons ago, at National level - winning some. It's a non issue. You're analytical/logical side will serve you well. If you have a deficit on the social side, really, many have worse deficits on the social side in trials and they are not even on the Asperger's spectrum. I've met many.
Forget about it. No one asks, no one cares, it's not a detrimental thing, it isn't checked, it can't be refused, it isn't identified, it doesn't exist as a barrier to entry. Really, forget about it. Asperger's isn't a disease, it's not a medical problem, it's not a medical issue, it's a condition. You can answer all of the questions truthfully without breaching guidelines. It's only if you get overzealous and you yourself classify it as something that it isn't in the interests of being over honest.
You're not being deceptive, not once. But to point this out, it's like filling in 'receding hairline' against 'Medical Conditions,' It's NOT a medical condition. It's who you are with no disease, no impediment, no detriment to safety, no recent condition, no detriment to performance, no inherited condition.
Finally, you already have a road license, which requires a greater degree of disclosure. Bear in mind that many people with Asperger's can't even ride a push bike. If you have been diagnosed as Asperger's then you are clearly on the lowest end of the spectrum. This is awesome. It usually means you get the advantages of Asperger's in the way your mind works and very few of the social inadequacies. If I could, I would recommend this for anyone, I really would, it's fantastic if that's where you are on the spectrum. You have a road license? Trials is a HUGE step down in legal requirements compared to a road license. You're overqualified already.
Asperger's is NOT a thing that has any medical negative and does NOT qualify against their questions unless you make it so. You don't need to mention it, not because of honesty, but because it doesn't warrant a mention and it's irrelevant as an answer to the questions asked of you.