I've been looking round the engine for tell tale signs of what could be causing the fuel to drain back to the tank and I came across this...IMG_20150607_17947.jpg
The pipe to the filter is metal but where it changes direction towards the foot well there is a tiny piece of moulded plastic pipe with a shaped rubber pipe over the top of that. This doesn't seem right, and the fact that the rubber pipe can be turned where it meets the moulded plastic can only mean that this has massive potential to be one of the points where air could be introduced. Times this by 4 as the feed and return pipes have both got the same pipe layout it looks like there is multiple areas where air could get in. Surely these pipes should be moulded plastic only?