and if your cleaver you could get a diffent tank fitted and run it on red diesel 
I'm afraid that the Webasto heaters don't work well on red diesel. The long boat forums are full of problems with these coking up with carbon very quickly and needing a regular expensive service and decoke every year. Long boats are fueled with red diesel.
Ordinary road diesel is fine, so its either the dye or the poor quality of fuel that is to blame.
From what I've heard, Ionics are the only ones who have truely got to grips with the Webasto Diesel heater for window cleaning, although I believe the lastest Pure Freedom unit is much improved. It would be interesting to see a picture of the inards of this Grippamax unit.
What would you like to know Spruce?
Not that I know 100% of its workings, but I may be able to assist...
Indeed, red diesel causes issues with them, and invalidaties warranties also?
The problem many users complain about is the battery going flat. This is due to the unit switching off when it has reached it's highest temperature. When a Webasto starts up it draws about 14 amps of current for a short period heating up the 'glow plug'. Once is it fired up it draws about 2 amps. Initially, as the unit is heating up, it runs at 'full throttle'. Once the temp of the heated water gets to a preset heat, say around 65 to 70 degrees, it will reduce the heater output and run at 'half throttle' and uses about 1.4 amps of current.
If you are standing talking to your customer, the water temperature will continue to increase until the unit switches itself off at around 90 degrees as you are not 'drawing any heat off.' Once you go back to cleaning, then the heater has got to restart and use that heavy current draw again. It will be in effect, cycling.
Ionic use a pressure relief valve from what I understand. If you, the operator switch your tap off, the pressure will build and the pressure relief valve redirect the hot water meant for your brush back to the main tank. But your pump will be running all the time.
The other issue is that a heater working in the 2nd heat phase (half throttle) will be supplying less heated water to the brush head as the temperature control valves will be adding more cold water to achieve your temperature requirement. So you need to find an ideal way of bleeding that extra heat off to keep the heater tickling over in that second phase with lower fuel costs.
A furnace (boiler) that keeps cycling on and off will coke up quickly, but one that runs efficiently will tend to remain carbon free for much longer. This cycling is another issue the long boat owners find a problem, especially when they are just heating up hot water and aren't using the central heating system. The internal heat exchangers in their calorifiers won't 'zap' the heat produced fast enough.
Added.
The primary use of these heaters are as block heaters; heating up and circulating freezing cold water in freezing engines, warming them up to normal operating temperatures to make starting easiler with less wear and tear. They then added an additional feature that switches on the vehicles internal heater fan to warm up the cabin and defrost the windscreen. In this application, they work very well and reliably. Using it as an on demand water heater in our business isn't what is is designed for.