@V8mini CanI ask where you have the new figures from? (Edit) Obviously form the web site.
AS mentioned RCI are every stubborn when it comes to changing existing contracts.The contracts have changed but the early ones said you could change it if more than 3 months to the end of the term.
Looks like you are in the same position as @askho so you should be able to change.
From the site:-
You can change the mileage and terms of the agreement at any time until 3 months before the end of the hire term. Any new monthly payment will reflect the prices shown. If the revised payment is greater than the original payment you’ll be charged for any additional costs up to the date of the change to the terms. Excess mileage is charged at £11.25 (incl. VAT) per 100 miles. In the event that the agreement is terminated additional charges may apply.
It is the bit in bold “prices shown” that you need to bring to RCI attention. Good luck.