Nigegilb,
The Nimrod has a similar leak detection system in the area of the double skinned pipework in the crew cabin. Any leakage into the interspace will drip/run into a bottle below the lowest part of the pipework and operate a floatswitch with a light on the eng's panel. Immediate Actions: stop AAR. If the bottle overflows, the surplus fuel is then directed into the interspace in the fuselage tanks and is then vented overboard at the back of the fuselage. Neat system.
You probably know more than me on this subject, so I'll ask you this: is double-skinned fuel pipework mandatory only when the pipes are inside the pressure shell, i.e where the crew and pax/cargo share the same air?
If so, this will explain why MOD 715 on the Nimrod didn't double-skin throughout the bomb bay, which is outside the pressure hull.
Regards
Ed Set