A big thank you for the reply and its similar to the formulas that I have stared at and through for most of today:-
"For this simple type of loan you have to solve the following equation:
Where MR is the monthly repayment, N the number of periods, r the
percentage APR, and 12 the number of periods in a year."
The bad news is that I do not have the MR and want to calculate r.
But thank you anyway, was almost saved :-)