I have no experience with financing purchases in the UK, but if it's anything like here, the dealer may act as agent for a number of lending companies. It's to their financial benefit to write a contract at a high APR if they have a customer who can't get a better line of credit elsewhere. However, they probably also represent other financiers who can do better. Tell them 16.7% is too high and see if they will take your application for another lender.
Meanwhile, be shopping for credit elsewhere... through your own bank or credit union (if you have one) or even other lenders you've already dealt with. In the time it took me to type this, I see some of our UK members have posted some good suggestions for possible sources.
One of my credit card companies just offered me a vehicle loan at a reasonable rate, which may help in negotiation once I receive the notice from Triumph that the bike has actually shipped. However, based on my credit record and previous experience with GE Money Bank (whom my dealer uses), I expect I'll get a slightly better rate through my dealer anyway.
That's assuming I decide to finance part of it at all. As gaptooth says, it's easier to buy money if you can prove you don't need it.
I've been scrounging and liquidating to have enough available to pay cash, but that doesn't necessarily mean I'll do it for the whole price. Sometimes it's better to have a bit of secured debt at a low APR and keep cash in reserve, so you don't have to put unexpected expenses on a high-rate credit card later. As Jon recommends, I avoid contracts with pre-payment penalties, so if I eventually do decide to pay off the note early, it's not so painful.
