I have a sheet which calculates the values for maintenance on each
site we visit as follows;

Cost of mainteanace upto and including five items £20.00 anything over
five items is £15.00 per item.

i.e. 10 items

5 = £20.00 + 5 * £15.00 = £75.00 + £20.00 = £95.00

15 items

5 = £20.00 + 10 * £15.00 = £150.00 + £20.00 = £170.00

Each site is different, the initial cost covers arrival, What is the
formula?

Regards Terry :-)