Scale rates for subsistence only apply to employees - not the self employed. Provided you are an employee then, as long as you have incurred subsistence expenses while travelling on an allowable business journey, your employer will be able to make tax and NICs free subsistence payments up to the advisory rates. In April 2009 these were as follows:
Away for 5 hours - £5
Away for 10 hours - £10
I don't think these rates have been increased.
There are a number of qualifying conditions that need to be met in order for employers to pay these rates and they also need to have a dispensation in place.