Pre heater when using Bio Diesel

I've been using Bio Diesel in my car (VW Bora TDI PD) for the past 6K miles.

Tend to go with a mix of about 45L Bio and 5L Fossil. Sometimes the full tank completely Bio.

The car runs fine and I don't want this thread to descend into whether Bio Diesel should be used or not. There's enough threads on that.

The car runs great on BioD though I can tell that there's a slight hesitancy when turning over on start-up.

I know this is due to the viscosity (and I have also changed the Fuel filter) of the BioD but was hoping I could fix this with some sort of pre heater.

There seems to be a lot of heat exchangers on eBay (some home-brew), but my understanding is that these can only heat the fuel once the engine is warm as it uses the coolant to heat the fuel. Kind of defeats the object for me as I want to heat the fuel on start-up, so (I'm guessing) it would have to be connected to the battery/glow plugs.

Anybody know what I should be looking for? Also, does it make a difference if it is installed before or after the fuel filter?