Paying back over 8 months, instead of 12 months, yields an even greater APR (8 months @ £60 = £480 paid back).
I found the following site:
http://www.whatsthecost.com/loan.aspx
..... and played around with the figures, until I reached:
If you borrow £300 over 8 months, with an APR of 142%, the monthly repayments will be £60.04 per month, which is close enough, so the true APR is approx 142% ........