HMRC have published advisory fuel rates to operate from 1 September 2013 until further notice report Harris & Co chartered accountants Northampton as part of their tax advice in their accountancy services for small and medium sized businesses.
For one month from the date of change, employers may use either the previous or new current rates, as they choose. Employers may therefore make or require supplementary payments if they so wish, but are under no obligation to do either.
The table of rates are available from HMRC.