I stated earlier that the MoD has been briefing that dual wall pipes are difficult to maintain (seals) and are more difficult to detect fuel leaks than single skin systems. I assume the last bit was said without a trace of irony. Well this is the reply I received from an experienced engineer
"No problems at all really. The maintenance would require 2 couplings, (one for the inner pipe, one for the outer) whereas on a single wall pipe you would have one. The outer pipe then has "overboard" drain points so that should the main pipe seep / weep / fail, the secondary gland drains the fuel overboard. These drains are engineered in such a way that if one is dripping then you know to within a small area where the problem lies."
I note the MoD briefing did not mention cost.