Sorry, it took me a while to get around to posting because I was laughing too much.
The critical point to convey to this mathematically challenged individual is that it matters not how little or how much any asset is used but how much so for business purposes: it is the proportion which counts. Say I have 5 vans, I christen Monday, Tuesday, Wednesday... and I use them only on the allotted day and only for business purposes. Have I used 500% of my eligible allowances? No. When you are claiming 95% business use, you are effectively saying that, whenever you use that vehicle, 5% of the journeys are non-business. The motorbike 50% of the time it is actually used. If you took the bike out only 60 times a year but 30 journeys were business-related, then half of your costs would be eligible.
You do need to be able to justify your business use. One might wonder what carpentry tools could be carried on a bike. (This practical issue doesn't stop you from using your bike to visit your accountant, solicitor or website designer or stationer - unless perhaps you came to pick up a filing cabinet). But I note that there are several city-centre car parks with height restrictions or similar where using a van would be impractical. Perhaps this is where a smaller vehicle comes into its own. I trust you attend each visit to the taxman on your bike, complete with leathers and a winning smile.
We must always assume that you are conveying the logical thrust of the HMRC officer's contention with sufficient accuracy. I almost hope, for his or her sake, that you are not. Beware that he or she is always entitled to come back with another reason, or reasons, why they think you cannot have the relief you've claimed.