My attempt so far (which does work) is

**=SUM(((i4*29.64)+(j3*32.15))/100)*1.05**

i4 contains a cell returning the difference between the last reading and today's

j3 contains the results from a cell calculating the DATEDIF function elsewhere on the sheet from the last bill date to today (using the TODAY() function)

29.64 is my unit cost in pence.

32.15 is my daily charge in pence.

1.05 is the VAT on the whole thing (unless Rishi has a change of heart soon).

Any comments or suggestions would be very welcome.