I think that you have just made an minor error in converting the units.
Your equation.
New Total Mass = Old total Mass + Change in mass
Should read New moment = Old moment + Moment change
But your figures below are correct
(47 800+m)*16.805 = 47800*17.05 + (16*m)
Answer= 14 547.8kg
If you now use the conversion factor 1 Imp Gal = 4.546092 litres as given in the CAP 696 you will get the correct answer.
14547.8 kg divided by 0.72 SG = 20205.27778 litres
20205.2777b litres divided by 4.546092 litres/Impgal = 4444.54 Impgals