Iain,
As discussed I suspect the baffling in your tank may not be the best and does not slow the the rush of fuel to the front of the tank when you go down hill - particularly as the filler is so far forward.  As you have indicated some sort of one way valve in the long filler pipe (I think yours is reduced to about 2" dia) that allows fuel to flow in but not out might be the go - this might be a simple as a spring controlled flap.
Another alternative could be to run a rubber pipe from the filler hole in the actual tank to the rear wheel arch and put a filler there.  That long pipe you currently have would not exist and the filler will always be higher than all sections of the tank even when going down steep hills.
Garry
				
			 
			
		 
			
				
			
			
				REMLR 243
2007 Range Rover Sport TDV6
1977 FC 101 
1976 Jaguar XJ12C
1973 Haflinger AP700
1971 Jaguar V12 E-Type Series 3 Roadster
1957 Series 1 88"
1957 Series 1 88" Station Wagon
			
			
		 
	
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