I'm not so sure about that, the builder. I think your calculation assumes teh amount owed is always £449 but because the money is being repaid over a 3 year period the average balance is much lower.
My figure of nearly 47% may very slight;ly out as I used a monthly repayment whereas in practice it's weekly.
But the actual APR may be a little higher if there's actually 157 weekly repayments rather than 156 (since some repayments have a payment at the start of week 1 and another at the end of the final week, meaning an extra week's payment).
Anyway the APR remains at 45-47%