Mark,

When you say fit a tell tail, is a side light bulb enough.

Do you connect it across the fuel pump connector or use the live and a separate earth & then the earth and a separate live feed. (a lot of wiring I suppose doing it that way)

I'm just thinking out loud if you connect it up across the plug you still need to test each side of the circuit and moving a connector could cause it to remake the circuit/connection.

Two bulb holders colour coded could possibly help identify which circuit broke down without testing both (well that's how I see it).

Hope that makes sense.