I have been able to calculate the reduction in growth where there is a lump
sum investment and have used (1+RATE(120,-600,86144,,1,3.5%))^12-1 where the
investment is made by regular monthly payments but cannot find a firula that
works where there is an initial investment and regular monthly payments

Say initial investment £100,000
regular investments £500 per month
Capital at the end of 10 years £210,000

What was the growth rate?