Feck me..........i only get that without towing!!
I tow up to 2 and a half ton loads quite often with my 110 td5 and I always get between 26 - 28 mpg towing or not. The only thing that seems to make any difference to the mpg is when I go above 60 miles an hour (not towing of course) then I'm lucky to get 22 mpg.
No petrol engine can compete with a diesel for towing, the high torque at low revs on the diesel makes it more economical. They are not many petrol lorries around are there.thats good going! dad towed my 90 on a twin axle trailer behind his 90 and used £50 of petrol to travel 50 miles!