We couldn't find anything that helped us work out our fuel capacity and rate of use, so here are our conclusions, based on John's calculations and my length of stick.
My dip stick is marked at 5cm intervals and by measuring before and after journeys plus some clever estimating, John calculates that 1 of these units equals 17.5 litres. He estimates the tank capacity at 200 litres.
A combination of dip stick before and after plus filling the tank from the diesel pump reveals that Patience uses an average of 0.5 litres per mile or 9.3 mpg. This will vary according to speed, winds and current but should hold good for an average journey at 4mph.
Do you agree?