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.
You really know your stuff Spruce, well done mate.
The trouble is that a lot of these units were jumped on by the suppliers and modified to suit our application, unfortunately with mixed results. The Webasto is a very good block heater and does OK as a supplementary cabin heater for vehicles such as the Range Rover Discovery/Disco.
On another current post, Archer has stated that he has had good service from the Hurricane diesel heaters that Brodex and a couple of others supply. These are specifically designed to operate in the marine industry, and their design seems to equate to a reliable performance in that industry. It would appear that they work well in the window cleaning industry as I haven't seen one thread of anyone complain, but is that because there are so few of them in our environment? I don't know. A basic Hurricane unit costs £1000.00 more than a Webasto kit, so one can see why the Webasto is more popular as the furnace source for a diesel water heater. I see Hurricane are now building a specific diesel heater for our industry - I haven't found a price yet though.
Susan is right, when she says that the excess heat could be channelled back into our WFP tank. However, that would be more costly to run the unit as that heat would mainly be wasted as most of us carry extra water incase we need it, and most of our WFP tanks aren't insulated to prevent heat loss. It would also mean another plate heat exchanger, pump and digital temperature controller that would operate the pump once the temperature of the water got too hot. This would all add costs to an already expensive unit.
Pure Freedom tried to solve this cycling of the heater by incorporating an 11 liter header tank that would be used as a heat store, but as they then followed Ionic's example of 'bleeding' excess heat back to the tank, we can only assume that the 11 liter tank wasn't a satisfactory solution.
When I first looked at this my intention was to warm the hoses during winter to keep them supple. However, over the years our minibore hose has better flexibility in the winter than the original lot did, so my interest in a diesel powered heater was given a lower priority.