To answer Sandy's original question:
You cannot just take the "premium" paid for a loan and call that the interest. I calculate the weekly interest rate to be 4.96% (to 3 significant figures). This translates to 1,139% annual interest rate. This is a mathematical answer, ignoring legal definitions.
This is high, compared to say a 25 year mortgage secured on a property, but I can see why people are happy to use money lenders for low value, short term, no-fuss loans.
So you can check my result:
The amount outstanding at the end of each week:
W1) (10 x 1.0496) - 1.30 = £ 9.20
W2) (9.20 x 1.0496) - 1.30 = £ 8.35
W3) (8.35 x 1.0496) - 1.30 = £ 7.47
W4) (7.47 x 1.0496) - 1.30 = £ 6.53
W5) (6.53 x 1.0496) - 1.30 = £ 5.56
W6) (5.56 x 1.0496) - 1.30 = £ 4.54
W7) (4.54 x 1.0496) - 1.30 = £ 3.46
W8) (3.46 x 1.0496) - 1.30 = £ 2.33
W9) (2.33 x 1.0496) - 1.30 = £ 1.15
W10) (1.15 x 1.0496) - 1.30 = 10 pence overpaid
Taking 52 weeks in a year, this equates to an annual interest rate of ((1.0496 ^ 52) - 1)x100 = 1,139%