Guppy: I've got a book that lists that Mossie as having flown through a plume of cascading (and burning) petrol after it shot down a LW bomber at night, rather than a V1 (which would have more shrapnel damage then burn damage, I'd think!)
Fire: Maybe the fire is that incendiary rounds are imbedding in the wooden frame and setting that on fire. Keep in mind that the reason the Hindenburg went down so lightning fast was probably the paint on the outside of the skin. Paint, glues, adhesives, all the chemicals put into building a mossie, are almost all flamable. Once flame gets going it can rupture fuel lines, melt protective coatings, whatever.
Might be that HTC made it light up because of this, and just decided the best way was to make the fuel takes a little more vunerable. I NEVER see fire coming from the wings, wing roots, it's always the fuselage. That makes me think it's not a fuel tank issue (otherwise each fuel tank could catch on fire, and none ever do but the fuselage)