How do I calculate monthly interest on a regular savings account?

I would appreciate if someone could provide me with a simple, yet implementable, formula to calculate the monthly interest on a UK regular savings account.

For example, I would like to invest £250 every month for a period of 12 months at a fixed rate of 4% (net) per month.

I look forward to applying your formula.

If the balance sits for 1 month I would take the balance at the time say £250 and times that by 4% that will give you the interest for the full year then devide by 365 to get a daily amount. Once you have done this times it by the amount of days in the month and that will give you the amount of interest it will generate for 1 month. you will have do do this for each month as the balance will always change if you keep adding £250 each month.