but what does your contract say?
Do you get 24 pounds per hour net, regardless? So if the revenue decide to change the tax treatement of sector pay, do you still get 24 per hour (i.e the company pay the extra tax). If so then grossing up the sector pay is a reasonable method to generate a comparative salary.
However if the revenue decide to tax the sector pay, and your gross amount stays the same, and your net falls as you pay the extra tax, then grossing it up is a lie.