How do there work out the APR 0f 1737%

It's the annualised equivalent interest. Normally it's calculated if the normal interest period is over a shorter duration e.g. for a payday loan.

You can google APR calculator, there are several you can use or look up the general formulae and use your calculator.

Because they rely on you having the loans for a very short time they charge extortionate rates of interest. If for any reason you can't pay the loan off you end up in a nasty spiral of debt very quickly. Only ever use these as a last resort and if you're sure you can repay on time.

Whichever way you caluclate it it sounds like a financial con!Do you agree?

The APR is exceptionally complicated. the bottom line should be how much money do you need and how long do you need it for. If you need ?100 for 30 days then sites like Wonga, iDosh, QuickQuid and the like would charge you ?30 I believe