Credit card Interest?

Question

I got charged interest on my credit card this month - can anyone tell me how they calculated the amount?I was charged £19.94 interest.Here are other figures on my statement -

Available credit - £949.34

Balance carried forward - £554.84

New items - £445.82

Closing balance - £1,000.66

My previous months closing balance was £554.84. I had paid £354.84 of it and I was just just 1 day late I think with paying the final £200.

The cards annual rate on Purchases is 19.95% p.a. The late payment fee is £12.00 but I don't know if this applies as I had paid the minimum payment.How did they calculate the interest?