I found the optical switch first, at first I thought I would only need one sensor in the reserve tank that would turn off the pump when the tank was empty, however the more I thought about it the more I realised that I needed to put some slightly more intelegent circuitry in.
At the moment, when the car is turned on, the circuit I built checks the sensors in both tanks and if there ir fuel in the reserve and the main tank is below the level or the fill tank (I decided to have the reserve tank fill from the main tank so I only have one filler hole) the filler sensor is therefore below the level of the overflow pipe so I don't just pump it round in circles..
When the sensor in the main tank registers as full it activates a relay that breakes the circuit to the pump until a reset button is pressed or the iginition is turned on and off (orginaly it just turned the pump off, but if the petrol sloshed arround too much the pump would keep cutting in and out, not a problem for an LED, but a motor doesn't like this sort of start-stop) I may at some point put a second float switch in to do the reset automaticaly but I'm going to see how it goes as it may not need it and I have a guage in the main tank that tells me how full it is.
As for the buffer cuircuit, I wan't sure I just read that one was needed, did a bit or research (read google) and found that a simple transitor cuircut was all it needed then I though long and hard of my old GCSE technology course, but that didn't help.... and did some more google reserch to find a very basic cuircuit.
So far I have tested it all but only with water. I have only just got my reserve tank from the fabricator (it sits behind the drivers chair and adds an extra 15L capacity to the 10L cylindrical tank) I bought some tank repair (motorbike suppliers sell it to seal the inside of tanks) it is an epoxy resin that sets to form a pertol proof skin, I seal the fluid level sensors into the tank by painting some of this onto the threads before tightening up, and with the float sensor I just tightened it up, the sulphone washer squashes to form a lip on the inside and outside of the tank no leaks yet..... The curve of the tank is barely negligable compaired to the size or the hole that needs to be cut... If the curve is too much your tanks you could always use a hammer to create a flat spot.