Whether 8.5% flat rate is any good depends mainly on the amount of the loan.
Smaller loans usually have a higher APR because the admin and other up-front costs make up a higher proportion of the total repayment.
18 months ago I got a 36-month car loan for about £5,000 at just over 9% APR (just under 5% flat) - and that was from a main dealer (these figures are from memory, but they're about right).
The first online lender I looked at (AA) is offering 6.8%APR for the same thing; and smaller amounts up to £3,000 are 15.9% APR.
So a flat rate of 8.5% sounds expensive; as conmankiller says, that would
mean an APR of 17-18%, which I don't think is very competitive.
If the offer's from a dealer who can't or won't budge on the finance, try to haggle for a reduction on the windscreen price to compensate.
Also the risk of a borrower failing to keep up repayments can be a
factor (increases the APR), so you might want to consider what sort of
borrower the lender is aiming at, and whether the lender would consider
them a good risk or a bad risk.
If you shop around for a loan, you'll find lower rates, largely because the kind of people who shop around:
1. are careful with their money, and will probably be good payers to the end of the term (less risk)
2. are also looking at deals from a dozen other lenders, so the deals have to be competitive
At the other end of the scale is the "sub-prime" lending market, lending at very high rates to people who (they hope) won't look too closely at the small print or care how much the loan costs as long as they can have the money.
Loans from dealers occupy a middle-ground - simpler than arranging a loan by yourself, but not always the best deal, but if you've done some homework beforehand, you needn't get ripped off.