"PJOS11" wrote:
If I am invoicing say £1000. Seven hundred of this
maybe charged at 17.5% and the remaining £300
would be charged at 5%.
[....]
£700+17.5%=£822.50
£300+5%= £315.00
Total £1137.50

If the tax rule is as simple as the above, the following
should suffice (assume the invoice amount is in A1):

=A1 + 17.5%*MIN(700,A1) + 5%*MAX(0,A1-700)

However, responses from others lead me to suspect
that your description of the tax rule is misleadingly
over-simplified. GIGO.