I think you would want it to ensure the excess fuel is returned to the tank in use. Unless you ensure never to fill the tank in question to the brim. And avoid idling/small throttle opening with attendant high fuel bypass for extended periods Or always drive flat out so there is no fuel return

Oof, Le Mans wheels! But £1400 for 4. And what size is 14.5x6.5...?  https://www.ebay.co.uk/sch/i.html?sid=godinsltd&_pgn=1&isRefine=true&_trksid=p4429486.m3561.l49496

Naughty boy 

Wrong engine size and external slave not internal concentric……

Out of my budget at the moment, and very likely wouldn't fit my mk4, but there's a rather lovely looking GT style hard top and bonnet on ebay at the moment: https://www.ebay.co.uk/itm/376063269177?_trksid=p4375194.c101949.m162918

Some lovely modifications, and then someone has put those awful tiny foam pancake filters on the air intake!

John Below is an edited version of the later XJ fuel system, from SC Parts. The device you have is a changeover solenoid, which would give you remote control over the tank you are drawing from (item 37 in the diagram). The fuel return solenoid (circled on the return feed diagram) would control which tank excess feed is returned to by blocking/unblocking each route. I suppose the question is whether you would need this, or, given your likely rate of consumption, simplify the installation and pipe to one tank for your return fuel? Paul

https://www.carandclassic.com/la/C1828632 Interesting looking thing, with clear pictures of some parts including the SAH 'tite-a-turn' rear setup.  The inlet manifold makes me wonder if it's an 8-port head on the 1147 engine. But no idea really. If I was in France I'd go and have a look  

Thank you, Paul! Previously, I had general advice from several of the  gurus, including Colin who provided a diagram from the manual for a small aircraft, which had a mechanical, hand operated valve to switch between tanks, with non-return valves and a pump down stream of the either/or valve.    Ideal, I think, except that I don't want a petrol valve on my cockpit! I'd rather put the fuel lines outside for safety.    But this Jaguar valve is not 'either/or', but 'on/off'.  It won't switch between tanks by itself.     Two pumps, with non-returns, as in Paul's car would do it (or should!) but that's extra complexity and cost.   To use this  on/off valve, there would need to be two, switching in opposite phase.  More complexity. Now, I find that I may have misled myself in buying this on/off valve!     Jaguar fitted a THREE-way change over valve, part no. CBC4269  fuel return - solenoid operated 3-way type : while what I have has only two ports.    B*664r!   No matter, I'll look again.   Thanks all! John PS the Jaguar spares site, SC Parts, describes this as a "Change over valve" subtitled "Fuel return".  It's for a fuel injection model, which will have a return from the injector rail, but is there any reason that this would not do for the main supply to the  carburettors on my project ?  J.