So how much Diesel did it use?
Well we did about 240 miles (40 miles over the allowed 200 daily use but hopefully they won't notice and charge me 8p per mile on the extra).
We found a station selling derv at 138.9 and put in £45. Not used to the price meter going faster than the litres meter, it's a bit scary. So that was 32 litres.
Just before returning it, I noticed the fuel was a little lower than when I picked it up, so I glugged in about 4 litres of unknown diesel from a container in the shed that has sat there for about 5 years. Didn't smell off and van seemed to run OK back to the hire place.
So I think 36 litres all in, 240 miles, which is approx 30mpg. Which isn't bad considering I ragged it about the place like some kind of van racing driver and did 70mph on the motorway.
Realistically the fuel consumption is a little worse than that because the speedo over-read by about 10%, which is also bit of an arse because that potentially costs the hirer extra on any mileage over the daily allowance.
Have you fitted it yet? 
At the moment it's in the back garden under a tarp (also sat on a tarp so not straight on the grass) - got to figure out where to put all the units and get the plumbing and electrics sorted. I think it might be out there a little while yet.