The fire hazard is mostly when there is loose stuff, like leaves or previously mown (and dried) clippings, that can ride up onto the top of the mower deck. I happen to be in a neighborhood with lots of trees, and strategically located so that all of the fallen leaves from a couple of city blocks in two directions are blown by prevailing winds directly into my yard, where they seem to settle. (After my neighbors use their leaf blowers to blow them all into the street where they "have good mobility.") I get "leaf drifts" that resemble snow drifts from a blizzard, occasionally 7 to 10 inches deep. I don't try to "mulch through" anything that looks deep enough to ride up under the mower; but it's not always possible to tell exactly how deep a localized pileup is.

On most mowers of this style, the engine exhaust and muffler are located at the bottom of the engine compartment, just slightly ahead of the mower deck, so that any significant accumulation on top of the deck can be ignited by the exhaust. A contributing "feature" is that the completely enclosed engine compartment makes it impossible to see the area where an accumulation is most critical. My anticipated partial fix will be simply to remove the hood (bonnet), and probably the engine compartment side panels, so that I can tell if something starts to build up.

John