Ok, its done. One mornings work and one long test drive later and I think its working ok. The heat exchanger is an inner metal fuel pipe surrounded by a hot water pipe fed from the Land Rovers cooling system. The valve manifold will only switch to veg when hot enough (or I decide to turn it on). Plus if I dont switch it on it will happily run on diesel. I decided to have the heat exchanger before the lift pump and filter cos I wanted the veg oil to be nice and thin before pushing it through the system, I have lagged all the exposed fuel lines and the filter with some space age pipe lagging (all silver and bubble wrap type stuff) to keep the heat in. Time will tell
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It look a little messy under the hood and some of the pipes have unnecessary joins (on account of me re configuring it a few times) But I have plenty of hose left and will be making it neater once I'm confident all is well.
Again many thanks for the help...