I just installed the radio-controlled switch unit. I won't bother posting a pic as it's dark now, and anyway the installation is a bit temporary as I want to install the receiver unit in a waterproof box when I can get one the right size. But the connection method will remain the same, so if you refer to the above pictures, the remote switch unit is connected to the rear junction box, in place of the temporary rocker switch, via a short 3-core cable and waterproof gland seal. The three cores needed are 240v live and neutral to run the unit, and a switched live return to the existing junction box to fire up the pre-heater and whatever else I eventually switch on at the same time (e.g. a socket in the cabin for a fan heater to defrost the windscreen, and/or perhaps a 250w oil pan heater.)
Anyway, it works. I now don't need to open the bonnet every time I switch the pre-heater on or off, and I can switch it on from inside the house. The remote fob seems to work in most places in the house, and certainly anywhere near a window I can guarantee it will switch the pre-heater on or off. The remote has two buttons, rather like a car key fob, so one button for ON and another button for OFF. It would nice to have a loud BEEEP from the car to confirm it has switched on, but it seems to be pretty reliable. And yes my friends, I can indeed switch on the engine preheater from the comfort of my bed (he he!) and by the time I've got up and got myself ready for work, the car is ready too. :-)
The remote was quite cheap. It's this unit here...
http://www.amazon.co.uk/Channel-Wire...=safari+drives
That's it. The mod is done essentially. Winter is almost over it seems (17 degrees C today! Very unusual for early March) so I won't bother arranging the cabin fan heater until later in the year, as that is only of use in the frost.
Now I'll see how this all improves mpg. I'm fairly convinced that for the purposes of improving mpg I only need to run the preheater for 30 minutes or so - to get the coolant temperature to 50 degrees or so. It will be worth doing that even in the summer. At 25 or 30 degrees ambient it will only take 10 or 15 minutes at 2kw to get up to 50 degrees C, but starting the engine and driving off at 50C rather than 25C will make a big difference in mpg over the first few miles.
I will know more the next time I fill up the tank, but since I did the 'alternator delete' mod and now the pre-heater mod, my overall average mpg, as shown by the (7% optimistic?) VW dash display has gone up considerably. I topped up the tank and zeroed the mpg display just after installing the battery charger and unplugging the alternator, and I have done 500 miles since then: all short journies in the local area, about half for work and half not. The overall average is showing as 91.5mpg Imperial and it's still rising. A month ago I was struggling to maintain an overall displayed average of 75mpg. Even in the heat of last summer when my mpg peaked, I was bragging about averaging a displayed 83mpg. 91.5mpg in winter, albeit a rather mild and wet winter, is encouraging. I shall wait to see what the filling station pump tells me, although I may be waiting a while as the fuel gauge is still showing more than 3/4 full!
[Update: I finally got the remote control receiver/relay unit fitted into another of those exact same waterproof junction boxes and the box tucked away just nicely behind the air filter housing. Putting the receiver in a box enabled me to stretch out its little coiled arial so it is now about 3 inches long rather than half an inch long. The remotes now work MUCH more reliably from inside the house, and I don't have to worry about being near a window. I have one remote by my bed and one on my keyring. Great geek-toys these radio-controlled switches!]