When I leave my LWB GT parked overnight on the flat part of my driveway (it's OK on the slope!), fuel drains back from the carb, empties the inline filter and sits in the lowest part of the line.

Is this a symptom of a failing fuel pump or is there something else I should be looking at? 

I'm using a standard Pierburg fuel pump. The tank's not mounted strangely, the fuel line run is exactly the same as on my GP SWB which has no such issues, but does have a Facet click-clack pump fitted at the tank outlet.

Just occurred to me it could be an air leak into the hoses.  Although there doesn't appear to be a leak out, I'll check the hose connections before going any further with the pump!

If hoses to pump are ok you have a faulty valve in the pump so with modern ones a new pump with old ones strip and clean/replace

I've got an electric fuel cutoff between the pump and tank, would solve your problem and a good safety feature and minor security help.
Have you got a filter between pump and tank, could be a bit of crud holding the valve open, maybe worth blowing it through.

Thanks for the replies.  Yes, there's a filter in the line, all the hardlines and hoses are new, the tank's clean and lined.  I can't find an air leak, but one of the hoses is a fairly slack fit to the line and clamped up with a fuel hose clip.  I'll redo that joint properly, but I'm pretty sure I'm up for a pump rebuild.